Oh, Hannah. I really do think you're a lovely girl, so please don't think that anything I'm about to say means I don't like you.
First of all, you're not fat. You might be gaining a bit of weight, but that's because you're a growing girl. You should listen to your friends about this and not to Romilda.
Really, I just wish you wouldn't worry so much. Not everything's your fault. And things often turn out better than you think they might; for example, you did just fine on your OWLs. Not that that will matter much since you end up leaving school at the beginning of sixth year--
Oh, and Zach's a bit of a prat, really. You're better off as just friends.
As much as you'll want to be with your friends, especially to hear the latest news in the Wizarding World, spend time at home this summer. Do things with your family, especially your mother. You'll regret it if you don't.