This is the most important of all. You can beat rivals who are better than you on paper. If you have enough self belief and where applicable better individual spirit. Personal self confidence is built up if you feel accepted as a person, have been loved by your parents and are confident that others believe in your talents. But even this self confidence can be destroyed.
Even if you are without those basic securities, you can learn to develop self confidence. To keep self confidence high and rising we must treat any setbacks constructively, learning lessons from anything that has gone wrong. And we must direct any angry energy into building up motivation for our next challenge rather than attacking ourselves for our failings.
Avoid cockiness and arrogance which although often mistaken for self confidence are signs of lack of it.