Why does my weight keep coming back?

I'm 14 and I recently lost a bit of weight from a small diet I went on but it's one month later and I've put it back on. I have been sticking to the exact same diet I was already on. I have gone to do extra exercise and ate less but all it's made me do is feel hungry! My belly keeps getting swollen and sometimes it really really hurts. I want to get to a size 10 for my summer holidays and i'm currently a size 12 but I just can't get rid of it. What should I do? P.S. Don't tell me if It has anything to do with buying.