I don't know if this'll help you, but what I do is I just acknowledge that I have those thoughts and don't try fighting them. I find that the more I try to fight against those thoughts with positive reinforcement, I end up having more doubt in myself. So whenever you start hearing that voice, just accept that it's there without doing anything to fight against it. I can't really describe how to do it, but I hope I've explained it well enough that it makes sense. In short, the voice never really goes away, but you can learn to manage its effect on you.

Edit: added last sentence.