Some would call it positive thinking, some would say it is a form of self hypnosis, but it seems to be the kind of thing that can remove unfounded doubt. Imagining you were confident, and what it would be like. There is also the aim of focusing on what is to be done, rather than looking for reasons not to do things, when we can focus on what we can do. I've clipped the titles, but there is more detail on the page. There is always the big question, when these kinds of methods are suggested. What could we do if we had the confidence?
Self confidence can improve, through the recognition of it's improvement, but we have to try it out, and be prepared for a few lumps on the way, when we find out we've been overconfident.