The granddaughter of Anne of Green Gables' author Lucy Maud Montgomery has revealed that her grandmother killed herself with a drugs overdose at the age of 67. LM Montgomery, who died in 1942, is one of Canada's best-loved authors, and wrote 19 other novels as well as the hugely popular children's classic.

Kate Macdonald Butler, daughter of Montgomery's youngest son Stuart Macdonald, made the long-kept family secret public in an article for Canada's Globe and Mail. "I have come to feel very strongly that the stigma surrounding mental illness will be forever upon us as a society until we sweep away the misconception that depression happens to other people, not us – and most certainly not to our heroes and icons," she wrote.

Macdonald Butler was also prompted to break the family's silence by the heightened focus on Montgomery this year, which marks the 100th anniversary of the publication of Anne of Green Gables.

"Despite her great success, it is known that she suffered from depression, that she was isolated, sad and filled with worry and dread for much of her life," Macdonald Butler wrote. She said that Montgomery had to cope both with "her husband's mental illness and the restrictions of her life as a clergyman's wife and mother in an era when women's roles were highly defined".

The family, which was consulted before Macdonald Butler wrote her article, has never spoken publicly about the extent of Montgomery's illness before. Hiterto, it was generally understood that she had died from heart failure. "What has never been revealed is that LM Montgomery took her own life at the age of 67 through a drug overdose. I wasn't told the details of what happened, and I never saw the note she left, but I do know that it asked for forgiveness," Macdonald Butler wrote.

Montgomery's most famous literary creation was the redheaded orphan Anne Shirley, who is sent by mistake to Matthew and Marilla Cuthbert on Prince Edward Island, where she declares: "My life is a perfect graveyard of buried hopes." Macdonald Butler said the lament had always been especially poignant to her, as she imagined her grandmother must have felt the same sadness at times in her life.

"The fictional Anne went on to happiness and a life full of love and fulfilment. My grandmother's reality was not so positive, although she continues to inspire generations of readers with her books, which reveal her understanding of nature – both in matters of the heart and the world," she wrote. "I hope that by writing about my grandmother now there might be less secrecy and more awareness that will ease the unnecessary suffering so many people experience as a result of such depressions."