Overriding my belief

Usually, I start my journey with confidence, ready to eat one meal a day. The first weeks are easy; my body adapts nicely. I’m sometimes hungry, but I’m full of joy for what’s to come. And then a nasty thought finds its way back: “What’s the point! Done that, been there. You know you’ll end up fat again”. I start slowly to lose my grip on my goal, to feel a miserable loser, and of course, I end up “fat again.” I know it’s a thought created by my mind, but I cannot shake it. How to override it?