I am reading Aristotle’s Ethics at the moment, in it he said,

“It is by our conduct in our intercourse with other men that we become just or unjust, and by acting in circumstances of danger, and training ourselves to feel fear or confidence, that we become courageous or cowardly. So, too, with our animal appetites and the passion of anger; for by behaving in this way or in that on the occasions with which these passions are concerned, some become temperate and gentle, and others profligate and ill-tempered. In a word, acts of any kind produce habits or characters of the same kind.”

You don’t have a character first and then act.

You act repeatedly, and those actions form your character. Confidence is the same. Confidence is not a personality trait. It is a trained habit.