I know, that's so vague and yet sounds like every other book being searched for. But I've been trying for years to find this book and I'm losing my mind so I'm here out of desperation. Of course I can't remember the title or the author, but here's what I do remember:
I read this between 1998-2001, so it was obviously written before 2002.
I'm pretty sure it's told from the POV of a girl, but it might switch between her and the boy. They both visit a forest of some kind that might be another world. There's an age switch somewhere, like either they're adults in the real world and become kids in the other world, or vice versa.
They fall in love in the other world but I think don't remember each other in our world.
I'm pretty sure the boy is important to the forest/other world, like they have to save it from some evil thing (my brain tells me there was a fire or the forest was on fire at some point) and maybe the boy is central to that.
There was some sort of guide or creature in the other world that they befriend. Either he helps them save the world or he dies during the attempt, I can't remember. But he was friendly.
I feel like there were multiple trips to this world, not just one. I have a vague recollection of the girl trying to find the boy when she visits. And maybe even at one point he isn't there.
I think there's some kind of age progression because I remember them kissing and possibly more. It's vague. But for sure she was in love with him.
It's possible they couldn't go back at some point, either they aged out of the place or saved the place and that sealed them out.
The end of the book takes place in the real world, they're both adults. They might both have forgotten each other, but she sees his face or a picture on the news (or in a newspaper) because (I think?) he was in a car crash of some kind. She remembers him and everything else. She goes to the hospital and he doesn't remember her. He might even have had a family and been married to someone else. What I recall is that she doesn't tell him about it, she just lets him go but maybe after they talk as strangers (at least in his mind) at the hospital. The ending had me crying for days because she had to let him go.
I could be wrong about this, but I think his name was Danny. Hers might've been Allison (or Alison or Alyson).
I also think the title was maybe just one word and the author was female. Again, don't quote me on that.
If any of this sounds familiar, PLEASE tell me. Thank you.
EDIT: It MIGHT be Hexwood, and I might've mixed that book with The Time Traveler's Wife in my mind. Once I receive my copy of Hexwood and find out for sure, I'll update here. Thanks to everyone for the suggestions!