She crafted her own image with exquisite care, creating herself on canvas over and over again, always the paradoxically triumphant victim. The legend she built around herself is so powerful it inspires and intrigues half a century after her death. Because at its heart remains a mystery.

Her intense, troubled marriage to her fellow Mexican painter Diego Rivera is well documented. So are her lovers, male and female, the childhood polio that left her with a thinner right leg and the terrible accident that broke her spine and pelvis. She painted her abortions, her back operations, her physical suffering. Others have focused on her mental pain and defiant spirit – in books, exhibitions and films.

Yet, despite all of this, Frida Kahlo is oddly elusive. The woman who made dozens of self-portraits in the 1940s and early ’50s was not lying when she called herself “the great concealer”. She painted her own narrative in bold swathes of colour, perhaps hoping that nobody would dare read between the brush strokes.

“Kahlo is still an enigma,” says her biographer Hayden Herrera, “because she held back a great deal, and part of her creative energy went into inventing her persona. Everyone who knew her talked about her alegria [cheerfulness]. It was important for her to appear strong, perhaps in order to fortify herself. I suspect that she was much less happy than she pretended.”

“She was vulnerable,” says Salomon Grimberg, the author of two new books on Kahlo – one focusing on her still lifes, including some that came to light during his research, and the other centring on a previously unpublished interview she gave to a psychologist friend. “We’re all vulnerable. We create a self-image to feel right about ourselves, and then spend the rest of our lives trying to protect it.” The difference is that Kahlo’s self-images have huge commercial value: her 1943 painting Roots sold for $5.6m at auction two years ago.

In Mexico, Kahlo is known as the “heroine of pain”. “I am alone,” she confessed in capital letters to her diary. But publicly, emphatically, she denied this universal truth. “She couldn’t tolerate being on her own,” says Grimberg. Her house was covered in mirrors – on the canopy of her bed, on tables, on her wardrobe doors, even in the garden. Did her reflection provide comfort? She often drew herself by looking in the mirror; in one drawing she appears to be left-handed, because it’s a mirror image.

“Here I am sending you my portrait so you will remember me,” she wrote on her first known self-portrait, a pencil sketch from 1920. “As long as she was in the mind of others, she existed,” says Grimberg. “If she got the attention of other people, she mattered.” Grimberg is, unusually, both a psychiatrist and an art historian. “I was always interested in why people make art,” he says. “Art is no different from the symptoms that psychiatric patients have, except you can visualise it.” He has been writing about Kahlo for 20 years. He lives in Dallas, Texas, and completed his medical training in the States, but he grew up in Mexico – and a member of his family was on the medical team that amputated Kahlo’s right leg below the knee in 1953. “I grew up listening to stories about Kahlo,” he says.

In the 1960s he worked at the Galeria de Arte Mexicano in Mexico City. “Even then, a Kahlo still life was $4,000 – you could buy a small house in Mexico for that then. Her Self-Portrait with Loose Hair came up for sale for a little less than $10,000 and I tried to persuade my father to buy it, but he said, ‘What an ugly woman! Why would I want that in my house?’ And when he heard the price he said, ‘Are you crazy?’ ”

As a hobby at first, Grimberg began to make lists of which collectors owned which Kahlo paintings; as his passion grew, he became known as an expert, and in 1988 he worked on the Kahlo catalogue raisonné (the compilation of all her known works). Since then, people from all over the world have contacted him to ask him to authenticate paintings they believe to be by her. Very occasionally, something previously unknown comes to light, such as the painting Self-Portrait as Genital, from around 1944.

From talking to people who knew Kahlo and reading their letters to and from the artist, he has painstakingly arrived at new interpretations of her work. Her still lifes have received less attention, but Grimberg believes they yield vital clues to her inner life. “The self-portraits were created very intentionally, thinking of others and how she wanted to be perceived. The still lifes were Kahlo’s personal reflections that she did not want to share with other people.” In 1938, for example, she made a painting of a flower, Xochitl, at the height of her affair with the Hungarian-American photographer Nickolas Muray; Grimberg has discovered, thanks to a letter he unearthed, that Xochitl was Muray’s pet name for Kahlo. It’s no coincidence at all, he says, that the red flower looks very much like a vagina enclosing a penis. Flowers and sex are intertwined in Mexican colloquialisms: “coger flores”, literally “to pick flowers”, also means to ask for sexual favours. Five years later, in Flower of Life, the symbolism is different. The plant, a mandrake, has long been associated with fertility, and the “arms” are fallopian tubes from which the ovaries are missing. The lightning and sun represent male pleasure, and it’s pretty clear that the flower is ejaculating. From this we infer that, at the time of this painting, Kahlo was giving a man a good time, but miserable in herself.

Some of the still lifes reflect her perceptions of situations in which friends found themselves. In 1943 she painted The Bride Who Becomes Frightened When She Sees Life Open: a study of watermelons, a grasshopper, an owl and a curious little doll in a wedding dress. At the time, Kahlo was waiting for her friend Jacqueline Lamba and her daughter Aube to come and stay. In 1941, Lamba and her husband, the French surrealist André Breton, had escaped war-torn Europe to live in America, under the wing of the great art patron Peggy Guggenheim. Lamba had just had her first exhibition of paintings, and everything was great – except it wasn’t.

Lamba was having an affair with the sculptor David Hare, seven years her junior, handsome, wealthy and crazy about her. He was married too, and had a reputation as a womaniser. But he begged her to leave Breton and marry him. When Breton found out, he warned her: “If you leave me, I will destroy you.” Lamba – represented by the doll – was terrified. The beady-eyed owl represented Breton. The grasshopper, or katydid, was Hare. “In Mexico, owls are linked to the god of death,” says Grimberg. “They feed on katydids – insects that can bring terrible catastrophes. And the bright yellow colour of the table is believed to frighten away pests.”

In the event, Lamba did leave Breton for Hare, preferring to risk a jump into the unknown than continue in an unfulfilling marriage. Kahlo herself would not be, or perhaps could not bear to be, separated from Rivera, even though he could be terribly cruel to her. But she too took other lovers. In an interview with Olga Campos, recorded on September 9, 1950, when she was 43 – four years before her death – Kahlo said: “Everything that gives pleasure is good, and everything that hurts is bad. I have never held back from sexual activities… An average affair lasts only while it gives pleasure.” She also admitted: “Sometimes I think about the end of the affair when it is just beginning.”

Campos contacted Grimberg out of the blue, after he had curated a Kahlo exhibition. Campos told him she had been a friend of Kahlo’s and had been working on an unfinished book about the creative process, as a student psychologist. She had met Kahlo because she was a classmate of Diego Rivera’s daughter Lupe (from his previous wife, Lupe Marin). She was young enough to be Kahlo’s daughter, and they took to each other right away; there is a photograph of them cheek to cheek. She spent many a weekend with Kahlo and Rivera, and observed their relationship at close quarters – witnessing “chilling quarrels” between them, but also “great tenderness”. “When intimate, he would play the child and she the mother, or vice versa. Diego would actually speak in baby talk at these times. One always seemed to know what the other was up to, and on days when they did not meet, they wrote to each other to keep in touch.”

Kahlo “loved him unconditionally”, almost as if he were a parent; Rivera loved Kahlo but was “emotionally unavailable and continually unfaithful”. Kahlo thought jealously was stupid, but Rivera’s affairs tore her apart. Rivera approved of Kahlo’s lesbian affairs, but Kahlo was discreet about her boyfriends. She sought sex to take the edge off the pain of Rivera’s numerous betrayals. Feminist icon she might have become, but the truth is she lived for Rivera – she dressed the way he liked, painted the way he liked.

In the 1950 interview, Kahlo spoke plainly about subjects ranging from her early life to children. “If I had a son, I would like him to look like Diego,” she said. “The first time I wanted to have a son, I was 13. I would see Diego walk by, and I would dream of having a son of his. I would comment on it to my little girl friends, eating ice cream in the plaza of Coyoacan.”

But she never did bear him a child. She chose to terminate three pregnancies, partly for medical reasons. Her pelvis and spine fractured when the bus she was on collided with a tram; she was not sure if she would be able to bear a child, though one doctor told her she’d be able to have a caesarean without too much difficulty. So she had no children for consolation, or to provide a kind of continuation.

Instead she surrounded herself with friends, even when she was in hospital. At her physical worst, she transformed her invalid’s existence into a party: she would have a film screen brought in so that she and her visitors could watch movies; they ate, drank, smoked cigarettes. She painted the plaster corsets she had to wear in bright colours. She turned her illness into performance art, having friends peer through a hole in the plaster to see how she was healing. She showed up to her one and only exhibition in an ambulance and was carried in on a stretcher to her four-poster bed, which had been carted in specially.

She refused to fade away, to keep her suffering out of sight, and she refused to let it get the better of her. She battled with suicidal thoughts but was full of life, a tonic to friends who would pour out their own troubles to her while she listened patiently, pleased to relieve the tedium of being bedridden. When drugs kept the pain at bearable levels, she painted for four or five hours a day, the canvas propped up on the hospital sheets.

“She made herself a professional patient,” says Grimberg. “She immersed herself in this identity to the extent that her perspective was skewed.” She believed her parents had loved her more when she was ill with polio, at the age of six. She also believed Rivera paid her more attention when she was sick. Nonetheless, her physical sufferings were very real – and her hospital stays gave her plenty of time to ponder life and death. And then paint them. “Everybody looks for the meaning of life. Kahlo looked for the meaning of death,” says Grimberg. One lover, the Spanish refugee artist Josep Bartoli, recalled Kahlo saying: “I paint flowers so they will not die.” On another occasion, she said: “Fruits are like flowers – they speak to us in provocative language and teach us things that are hidden.”

In Pitahayas (1938-9), the pink fruit are cut open, needing to be eaten before they rot; the small skeleton to the side tells us that the time to appreciate life is now, before it is lost. “Originally,” Grimberg says, “Kahlo made the painting with a mixture of awe and playfulness. She painted the skeleton smiling, which is how it appears in early photographs.” Then Rivera told her he wanted a divorce, and presented her with the papers. “Mirroring her disappointment and depression, she replaced the skeleton’s smile with a frown, as it appears today.”

Many artists have painted still lifes heavy with the symbolism of mortality. But Kahlo seems to have been particularly obsessed with it. She told Campos: “I think about death very often; too much. I have wanted to die out of desperation. I imagine that if I were dying I would be thinking of Diego.”

“Everybody at some level feels lonely, misunderstood, hurting and isolated,” says Grimberg. “That’s what people identify with in her work.” “Painting is the only skill I have – and nothing else,” she told Campos. Plainly she was wrong. In putting the darker emotions we all experience into visual form, in insisting on colour and vitality and beauty, she communicated that life is always worth living – no matter how painful it can be.

Frida Kahlo: The Still Lifes (Merrell, £25), and Frida Kahlo: Song of Herself (Merrell, £14.95), both by Salomon Grimberg, are available from BooksFirst for £22.50 and £13.45 respectively, including p&p. Tel: 0870 165 8585