I feel really fat all the time?

It seems to me like you are just comparing yourself to others too much. If you are in ballet then you are always around super skinny people. That's not necessarily a bad thing, but I will change your outlook on how you feel about yourself. I used to have the same problem feeling overweight when it was clear to everyone I wasn't but I just wasn't happy with my body. I asked my mom for advise, and she gave me a good tip to put a bag over my head, cut to eyes in the bag and just look at my body in a bikini or something. It really shows you the real you, because you have this mental image of yourself that goes around in your head, but i promise this works! If you still aren't satisfied with what you see try figuring out where your having problems and try to fix it with a healthier eating plan and exercise. That could always be good. (: