“It’s hard to put a size on things that happen in your life,” Gladys McGarey says. At 102, it’s understandable she would feel that way. Shortly before she was 70, her husband, Bill, to whom she had been married for 46 years, handed her divorce papers. He had been carrying them in his briefcase for six months and said he would appreciate a prompt signature.

McGarey and Bill had not only been married for decades but were also close colleagues. They were both doctors, who had trained as general practitioners, and were among the co-founders of the American Holistic Medical Association in 1978. For the duration of their marriage, they had shared a clinic in Phoenix, Arizona, as well as having six children together. So, when the marriage ended – Bill had started a relationship with another colleague – McGarey lost not only her life partner but her business partner, too.

While McGarey still had “something to get up for each morning”, the pain of the separation was immense – “the hardest thing” she has ever faced. Harder, she says, than the two bouts of cancer she went through in her 30s and 90s.

“It’s not a matter of getting over stuff, it’s a matter of living through it,” she says now, speaking on a Zoom call from the house that she built, at Helene’s invitation, behind her daughter’s home. “If you can live through the issues that you have been faced with, they become one of your teachers.”

But it was a slow process. “It wasn’t until I was 93 that I really accepted it,” she says. And, clutching the air with her hand: “You get to that part where you can kind of hang on.”

McGarey is a presbyterian; her parents were medical missionaries, and she was raised in India till she was 15. In the car, a verse from the Bible came to her. “This is the day that the Lord has made, let us rejoice and be glad in it.’”

So she bought a licence plate for her car that read “BE GLAD”, to remind her that there was a lot in her marriage “that I really treasured”.

Looking back, McGarey can now see that her divorce was a critical and constructive turning point: “Prior to that I really didn’t trust my own voice.” Being dyslexic meant that at school she always felt slow. “I really didn’t think I had a voice,” she says, yet, “I knew I had a voice. After the divorce, it was no longer Bill and Gladys, it was Gladys McGarey MD. I reclaimed what I had, not just as Bill’s partner.”

“I don’t regret one minute that I spent as Bill’s wife,” she says. But, while he died in 2008, “My life has taken on these amazing dimensions that are completely not associated with him,” she says. “What I chose is working for me.”

McGarey never remarried. “People would ask if I was dating and I would say: ‘Why would I want to take care of another old man? No thank you.’”

She retired at 86, but continues to offer telephone consultations; she has an appointment after we finish speaking. She no longer has a licence to practise, but, as she puts it: “Nobody told me I had to stop talking.” Indeed, at 100, she delivered a TedX Talk – on holistic medicine and her central belief that she has “a colleague within each patient who does the healing, as I do the other part of it, which is what I know and understand in the field of medicine”. She has just published a book offering guidance for a well-lived life. Her hair, which she has not cut since her 40s, is neatly plaited into a white crown, ready as ever for work.

The Well-Lived Life: A 102-Year-Old Doctor’s Six Secrets to Health And Happiness At Every Age by Dr Gladys McGarey (Penguin Michael Joseph, £16.99) is out now. To support the Guardian, order your copy for £14.95 at guardianbookshop.com. Delivery charges may apply.