The human mind is an incredibly powerful and really weird contraption. If it believes something to be true, then it almost bends reality to make it so. This is most easily demonstrated with the placebo effect -- which works, even if patients know it's a placebo. Buddhist monks can alter their body temperature through meditation. Athletes can train for years in order to achieve superhuman levels of strength.

Unfortunately, that power works both ways -- if you believe that you're incapable of something, you will fail, even if you're otherwise capable. Most people limit themselves by believing that they're less capable than they really are. Most people give up when they could push through and succeed.