Edith Eger was 16 years old, crammed into a cattle truck, human cargo from Hungary headed for Auschwitz, when her mother gave her the advice that shaped her life. For most of the journey, her mother hadn’t said much, hadn’t cried or complained, but had instead gone inside herself. “That night,” says Eger, “she turned to me and said: ‘Listen. We don’t know where we’re going. We don’t know what’s going to happen. Just remember, no one can take away from you what you’ve put in your mind.”

For the next year, Eger’s inner life – cherished memories, favourite recipes, future fantasies – sustained her, even saved her. After liberation, though, it turned against her. Survivor’s guilt, buried memories and constant flashbacks held her hostage. A siren, a shouting man, a piece of barbed wire could hurl her back to 1944. Ultimately, Eger’s mission to understand her mind and utilise its power led her to become an acclaimed psychologist specialising in trauma. Her mother’s words have formed her life’s work.

Now 90, smiling and immaculate in vivid turquoise, she talks to me from her light-filled home office in La Jolla, California. Her next patient is due in an hour. “I do not believe in retirement,” she says in heavily accented English. “My patients are my teachers.” Life now is good. “I live in paradise with an ocean view from the front and a beautiful canyon view at the back,” she says. “I go dancing once a week. I live in the present and I think young. I’m kind of celebrating every moment.”

Eger’s book, The Choice, is an international bestseller and took 10 years to write. She began it after the birth of her first great-grandson, for her family to read. “I was hoping it would be in their living rooms, and they’d see me as a good role model,” she says. “Its reception has been the biggest miracle of my life.” But transporting herself out of her “paradise” and back to hell was not easy. “It was very difficult, but I think it’s the best thing I’ve ever done,” she says, “because, you see, the opposite of depression is expression. I was able to put it out there and cry and cry. With every page I lost 2,000lb of emotional weight.”

Eger’s story starts in Košice, Hungary (now Slovakia) with her parents and two older sisters. Her father, a tailor, was a lover of life. Her mother was more distant, prone to disappointment. One sister, Klara, a violin prodigy, studied in Budapest, where she managed to hide throughout the war. Another, Magda, was the “jokester”, the one with the attitude. Eger was the “invisible one”. “I was a very erudite teenager,” she says. “I had my own book club and was reading Freud’s Interpretation of Dreams. Why? Because my mother told me, ‘I’m glad you have brains because you have no looks!’” So an ordinary family, as imperfect as any other.

I couldn’t fight or flee in Auschwitz, but I still had choices

With the Nazi grip came curfews, yellow stars and evictions. Life tightened for Jewish families. One night in April 1944, soldiers pounded on their door and took Eger, Magda and her parents to a brick factory where they lived for a month with 12,000 other Jews. Next was Auschwitz. On arrival, Eger’s father was herded away with the men and her mother was also separated when the infamous “Angel of Death”, Dr Josef Mengele ordered anyone under 14 or over 40 to a different line. (“She’s just going to take a shower,” Mengele told Eger when she tried to follow her.) Eger never saw either parent again.

Her survival in Auschwitz is partly testament to the power of her mind. On her first night, while she was adjusting to the inconceivable, Mengele entered her barracks looking for “new talent”. He ordered Eger, a trained ballerina, to dance. Somehow, she closed her eyes and transformed the barracks into the Budapest Opera House. Somehow she earned a loaf of bread.

“In Auschwitz, we never knew from one moment to another what was going to happen,” says Eger. “I couldn’t fight or flee, but I learned how to stay in a situation and make the best of what is. I still had choices. So when we were stripped and shorn of our hair, Magda asked me, ‘How do I look?’ She looked like a mangy dog, but I told her: ‘Your eyes are so beautiful. I never noticed when you had all that hair.’ Every day, we could choose to pay attention to what we’d lost or what we still had.”

After six months, as Americans and Russians advanced, the Nazis began to evacuate the camp, and the sisters were forced to join the “death march” across Europe. When GIs finally lifted them from a pile of bodies in an Austrian forest, Eger had typhoid fever, pneumonia, pleurisy and a broken back. Healing her body took time – but in a year she was married to Béla, whom she met in hospital. (He, too, had lost his family, but survived in the mountains, joining the partisan resistance.) “At that time, all we asked was: ‘How can we be normal?’” says Eger, “and ‘normal’ meant getting married.” On her honeymoon, she became pregnant – against the advice of doctors who believed Eger too weak. Her daughter, Marianne, was a healthy 10lb baby.

But mental recovery took far longer. Neither Eger nor Magda talked about what had happened – not to each other or anyone else. Denial was their shield. “We felt that the more securely we locked it away, the safer we were.” Magda, Eger and her new family all emigrated to the US. Thousands of miles separated Eger from her past, but the memories and trauma came with her.

In The Choice, Eger describes her flashbacks – her racing heart and narrowing vision – in visceral detail. Once, in Baltimore, taking the bus to her factory job, Eger boarded the European way, taking her seat and awaiting a ticket collector. The driver yelled, “Pay or get off!” He got up and walked towards her. She fell cowering to the ground, crying and shaking.

I felt like an imposter as I hadn’t really dealt with my past

Though Eger refused to speak of her past to her three children, her 10-year-old daughter Marianne found a history book with pictures of the skeletal corpses piled in a heap. She asked her mother what it was and Eger had to run from the room and vomit in the bathroom. Settling in El Paso, Béla and Eger built a comfortable life. Béla qualified as an accountant and in her late 30s Eger began studying psychology at the University of Texas. Slowly, cautiously, she started to talk about the Holocaust and examine her experience, intent on learning how we survive trauma and what transforms a “victim” into a “survivor”. She took an MA, a PhD, then earned her licence to practise.

Specialising in post-traumatic stress (Eger objects to calling it a “disorder” as it’s a common and natural response to trauma), Eger began working with the American military. But her true breakthrough came when she was 53 years old. “I had a white coat and it said ‘Dr Eger’, but I felt like an imposter because I did not really deal with my past,” she says. “I could not be a good guide to my patients or take them any further than I’d gone myself. For that, I had to go back to the lion’s den and look at the place where my mother was murdered, where I was so close to death every day.”

It was during this return to Auschwitz that Eger confronted a devastating truth, a memory she’d hidden even from herself. When she had arrived at Auschwitz and awaited selection, Mengele had looked at her mother’s unlined face, then turned to Eger and asked if this was her “mother” or her “sister”. Eger didn’t think about which word would protect her – she simply told him the truth. Her mother was moved to the other line – the line that led straight to the gas chamber.

“Until I returned, I was my own worst enemy,” she says. “I not only had survivor’s guilt, I had survivor’s shame. I didn’t need a Hitler out there, I had a Hitler in me telling me I was unworthy, that I didn’t deserve to survive. On that day, I allowed myself to be human – not superhuman and not subhuman. We do things the way human beings do and we make mistakes. If I had known better, I would have done better – I would have, believe me. But unless we acknowledge that we cannot change the past, we cannot really heal and live life.”

Every part of her experience has informed her work. “I studied it and I lived it,” she says. “There is a difference between all the knowledge you get from books and all the clinical experience – both of which I have – and the ‘life experience’. That’s what I use most. I help people realise that the biggest prison is in their mind – and to be free of the past means not to run from it or forget it, but to face it. I see my work as my calling. And I’m still not done.”

The Choice by Edith Eger is published by Rider Books, £8.99. Order a copy for £7.64 at guardianbookshop.com