~ I NEED SUPPORT DESPERATELY ~

Hi, I'm a 13-year old girl named Kary. I weigh Roughly 186lbs and I can't lose weight no matter what I do. I'm always embarassed to go to doctor physicals becuase my doctor always has people in trianing there. My doctor will always tell me to lose about 10 pounds but stay the same wieght even when I grow taller. I'm about 5'9" tall and just got my period so I only have about a year longer to grow. I'm sick and tired of having to wear one peice bathing suits and long jeans rather than bikinis and short like all my friends. I need support more than anything, becuase my mom supports me, but I don't really belive that she believes in me, becuase of all my failed attempts at weight loss in the past. I eat very healthy and do a moderate amount of excersize but I have a very low metabolism. My sister has a very hgh mteabolism and so its hard for me to watch her eats sweets and processed food without giving in even thoough I know it wont effect her but it will effect me. I do situps, lengths, you name it, but it just builds muscle and makes me look even bigger. My grad is coming up in June and its now July and I want to be a size 8 by then. Please help!