If we formulate limiting beliefs about what it's possible for us to accomplish, then our response will be to bind the actions we take. As a result we likewise limit the results we get. If we believe we're likely to fail, our subconscious will get to work to produce actions which support this. If the going gets tough (and it commonly does), we won't go the additional mile required to succeed; an initial failure simply supports our notion that we were going to bomb all along and we stop. On the other hand if we trust we're going to succeed, then we likely will.