This has been bothering me for years, I never can seem to find this fantasy YA book I read as a teenage. What I remember is that the protagonist is a young girl living in a medieval/post apocalyptic setting. She finds out she is special in some way, can somehow help her world have sunlight again or be less gloomy. There's an older woman, a witch/sorceress who wants her dead. There's also something about a powerful, magical amulet/necklace.
The young girl is a reluctant hero, she goes on a long journey. The book is older, from the 90s maybe even 80s. I thought for the longest time it was by a New Zealand female writer but I'm not sure about that.
Any help is appreciated!