I used to be bulimic too, and I guess you already know that a diet won't change a thing. You'll probably feel better with your body a short time then go back into the food thing like I did for about 6years. I don't know how old you are, neither what's you're story, you can talk about it if you want, but I believe that you will need to work on the deep problem to fix the whole body thing.
By the way, staying an A cup when gaining weight may be a sign of an hormonal imbalance ? I know a little about this but this doesn't sound normal to me. Did you reach a B or more cup the first time you got overweight ? My weight fluctuation through the year used to be more around 6kg (I can't do the pounds conversion right now sorry, but I think that's about 13lbs) and the first time I reached my maximum weight I was close to a D cup, then the years after from losing and regaining weight I was more of a C when at the same weight.
By the way, staying an A cup when gaining weight may be a sign of an hormonal imbalance ? I know a little about this but this doesn't sound normal to me. Did you reach a B or more cup the first time you got overweight ? My weight fluctuation through the year used to be more around 6kg (I can't do the pounds conversion right now sorry, but I think that's about 13lbs) and the first time I reached my maximum weight I was close to a D cup, then the years after from losing and regaining weight I was more of a C when at the same weight.