I personally think that i am fat! But everyone ...

Everybody already knows you're not fat, especially if your doctor told you so.  You have a problem with self-perception.  Maybe videotaping yourself turning around so you could examine your body from all angles and compare it to other people will help.  If not, you may need professional help of some type, like therapy.

first of all, if everyone has told you this, surely it's not true. but i do know what you mean, ive been there before so many times, and i know that just words arent usually enough.

 try googling "BMI" or "body mass index". this is a scientific system that should tell you. if you still dont believe it, try making a list of all the things that you DO like about your size or yourself. this will help raise your confidence and help you believe that you are beautiful and not fat.