If you don’t believe in yourself, then you believe in yourself. If you doubt your abilities and think that there is something wrong with you, then you must have a strong belief that you exist. You believe in yourself like you believe in the Easter Bunny, even though you may not believe in yourself.
There are those who believe that the Easter Bunny is only a myth, a story to entertain children. These non-believers consider themselves knowledgable about the world. They think they can discern fact from fantasy. They believe in themselves.
If you believe in the Easter Bunny, or God, or Buddha, or yourself, that is your belief. Some beliefs are right and some are wrong. If you believe in yourself, but don’t believe in yourself, then some of your beliefs are wrong. Unless it’s the other way around.