What was the name of this film?

I am trying to find a film made in the 70's or possibly early 80's, American or British. A woman who writes children's books is losing her mind in a little house in a kind of wooded rural setting. She thinks she is being stalked by a body double of herself. She misperceives and misunderstands the actions of the other real people around her. Ends tragically. Has anybody heard of it??