It is going to take some time to change you to go from negative to positive self talk.  However, this I know for sure.  When you practice writing down and repeating simple, positive affirmations over time positive thoughts will become second nature and your life will change.
Suggested affirmations or create your own:
I completely and totally love and accept myself.
I am free of guilt about my food choices.
I am grateful for the body I have and what it does for me.