Just one thing bothered me: she hadn't mentioned what happened with her getting her diary taken away by those two boys for days. I've noticed no inner guilt or anger or turmoil about it.
No grammatical errors caught so far, though, so very good!
Good idea, but not much originality. Will this diary of an eating disordered girl be unique in any way than the rest? And it seems to be all over the place. All of a sudden someone's reading her diary, then the next she seems to forget all about it and talk about this new diet she wants to start. I felt no emotion, no connection, no inner turmoil during that diary entry. You may be going for simplicity in this story, but even the simplest lines can have the greatest impacts on a person.
Take this idea and make it your own. And also make sure you understand what exactly runs through a person's mind when they're suffering from an eating disorder. Unless you're suffering from an eating disorder yourself, you don't really know what it's like. And I'll be your biggest critic, because not only do I suffer from disordered eating, but I also am close friends with girls who suffer from severe Anorexia Nervosa and Bulimia.
Good punctuation and grammar, though, so I commend you for that. It shows you're a pretty experienced writer. But I'd like to see more of a voice with this girl. This is her diary, after all.