I am sorry to hear that you have such a problem.I'd like to give you some advice on how to keep fit.
First,you should eat more vegetables and fruit and less meat.As the saying goes,an apple a day keeps the doctors away.Apples are very good for your health.You can get a lot of vitamins from them.
Second,do more exercise.Be sure to make it a rule to run for a few miles a day.That benefits your health phisically and mentally.
Third,just don't worry about your size.It is very normal for a teenager like you to be a little fat.Take it easy,and you can do it.
Finally,best wishes to you.