Why can't I lose ANY weight?

I am 12 years old "4'11" 118 pounds, 33-30-36 and for NO EXPLAINABLE REASON have been gaining a mive amount around my already 34 inch belly on!!!! I see the girls at school who are SKINNY and CURVY and you know what? THEY EAT JUNK! If they ate like me they'd be dead! They eat everything without exercise and are perfect, and I eat 800-900 calories a day (and never more than 30g of sugar, I may add) and exercise consistently and have literally cried myself to sleep every night about my shapeless, saggy, misshapen, flabby midsection!!! I just don't know WHAT to do! I can't even fit pants that don't have an elastic waistband. I AM DISGUSTING. I would like to lose at least 10 inches around my belly on. If you are wondering, no. I haven't had my period yet (I'm from a family of late bloomers, my mother got hers at 16 and my sister at 14, but she was still skinny before that.)