I too have a "fat voice". I have been working very very diligently to shut it up and I have found that good simple "no" responses work well.

The root of the problem is that you have to develop an intense relationship with yourself and figure out why exactly it is that "fat" is bad. Would it make you any less of a person if you were? You have to become a person inside and not worry so much about who you are on the outside. I know it sounds like therapist rabble, but seriously it has helped me.

Think of yourself as a child, visualize yourself in a mental state where you can just be in awe at the beauties of the world as opposed to the imperfections, because in those imperfections, thats where the beauty lies.

I have been binge/purge free going on almost four weeks now (its been exhausting) and I have found that my fat voice has gotten quieter and quieter, until, seriously, i think it took a vacation today. I was able to be normal and share laughs and live life to the fullest all why wearing my "fat" pants (we all know the ones, they make your butt look big and have never fit right, but you keep them just incase miraculously they fit one day).

I'm not sure if this rant has helped you at all, but I feel better. You can do it! Just remember, if you feel fat, chances are your mind is craving some other attention other places in your life, its just its way of getting your attention.