I thought and thought about this one. Then I remembered a quote of Mahatma Gandhi I like.
Man often becomes what he believes himself to be. If I keep on saying to myself that I cannot do a certain thing, it is possible that I may end by really becoming incapable of doing it. On the contrary, if I shall have the belief that I can do it, I shall surely acquire the capacity to do it, even if I may not have it at the beginning.