Ah, yes it's hard to change that negative little voice in your head, especially if you have grown up with people around you reinforcing it at every step it becomes part of who you are in the end and it's hard to imagine defining yourself in any other way. At some point you have to stand up and say "i'm not putting up with any of this nonsense" and giving yourself a firm talking to... and actually believing that the negative crap you are saying IS NOT really who you are. I found that if i challenge every negative thought that enters my mind, it does eventually help change it - it takes quite a bit of effort for a while though as it's hard to spot the negative voice but eventually it becomes a bit like a "whack a mole" contest at the fairground, spot the negative thought and swat it away to be replaced with something positive. Over time this technique really helped me free up my thought process and allow positive thoughts to take root. And the other thing is removing negative people, events, associations from your life too.
"I value myself enough to feed myself only food that will nourish me and make me healthy. I deserve, and will be, a healthy weight. I am not perfect and make mistakes, but these mistakes make me stronger and more determined to succeed, my downfalls will not beat me. I will make a difference in my own life and be in control of where i am going and who i will be 12 months from now, a week from now, or even who i am in the next 5 minutes."