From my own life experience I've found the best way to feel happy and fulfilled is to be true to yourself, and I wish I'd followed this more when I was younger.
Don't try to suppress your feelings because of what others around you may think. It just won't work in any case, and as you get older and more confident these feelings won't go away and will come to the surface eventually. Or else you'll go your whole life feeling unfulfilled or ashamed.
Many people here have struggled with the same issues, and have eventually realized that there's actually nothing wrong with wanting to be fat, whether in fantasy or reality, any more than it's wrong to want to be thin, or rich or famous or just a beach bum. They're all just different feelings and desires and no-one is more entitled to their feelings and desires than anyone else.
If people around you don't accept you for what you are then that's their issue and their loss, however you will find people that love and accept you if you look in the right places