Believing you can do something is certainly no guarantee of success. But believing you can't do it is, beyond doubt, a guarantee of failure.
The trick, I think, is to start out with baby steps to convince yourself that success is not just possible, but probably. These baby steps may be steps along the path to your specific goal, or they may be unrelated, things that give you confidence--perhaps learning public speaking, getting into shape, starting conversations with 100 women in one month, whatever.
