One recent evening, Sandra Demontigny tried to write down when she would die.

“I sat down in a corner with a candle next to me, just to create my own bubble, to think and to cry a little,” she said.

She had reflected on this moment for years, desperately hoped for it, fought tirelessly for it. But the words refused to come out. The form before her remained blank. How, exactly, does one decide when to end one’s life?

Canada’s French-speaking province of Quebec last fall became one of the few places in the world to allow a person with a serious and incurable illness to choose medically assisted death in advance — perhaps years before the act, when the person still has the mental capacity to make such a momentous decision.

And Ms. Demontigny — a 45-year-old mother of three, diagnosed in the prime of her life with a rare form of early-onset Alzheimer’s — played a pivotal role in lobbying for the change.

Some facing such a grave health challenge might have withdrawn. But even as Ms. Demontigny (deux-mon-tee-gnee) began losing her memory, she became the face of the campaign to expand the right to die in Quebec.

In front of health ministers and lawmakers, on talk shows, in countless interviews, she spoke of how she had inherited the Alzheimer’s gene carried by her family. She recalled how her middle-aged father, in the last years of his life, became unrecognizable and aggressive. She wanted to die with dignity.