It's been a little while since I read Wintergirls, and I don't have a copy in front of me, but when I read it, I actually didn't take the ending to mean that she gets better right away. I took it to mean that she was starting on the path to get better, that there was hope for her getting better. (And I was glad for that, because I need that kind of hope with my tragic stories; otherwise one could be content with real life.)

I admit that I really like Laurie Halse Anderson's books, so take all this with that grain of salt -- but if you ever feel like giving her another try, I'd recommend Catalyst, which I think is my favorite of her YA books.