Mary has always liked the "what-if" game; she plays it with herself when she's having a cup of tea, or listening to music, or any sort of solitary activity. She likes to change things that happened in the past just a bit and think through what the likely results would have been.Sometimes, she thinks about her hospitalization, and after a few rounds of the game, she realises it is one of those things that would have happened no matter what. Everything she tries to change results in it anyway: what if Matt hadn't been dared in the first place? Well, then, she probably would have starved herself to that point anyway, probably after Hogwarts, but she knows she would have. What if Matt had told her earlier about the dare? Even if he had told her immediately, it would still have been a convenient catalyst for her disease.
Mary isn't cured, she doesn't think she'll ever really stop struggling against the voices in her head, doubts she'll ever be able to eat without feeling a little bit guilty...but it's better, better than it ever was before, and it's getting better all the time.