Fire and Hemlock — Diana Wynne Jones

Polly has two sets of memories...

One is normal: school, home, friends. The other, stranger memories begin nine years ago, when she was ten and gate-crashed an odd funeral in the mansion near her grandmother's house. Polly's just beginning to recall the sometimes marvelous, sometimes frightening adventures she embarked on with Tom Lynn after that. And then she did something terrible, and everything changed.

But what did she do? Why can't she remember? Polly must uncover the secret, or her true love -- and perhaps Polly herself -- will be lost.