What competence has to do with it

Confidence that runs ahead of competence is a liability, and other people can see it. Confidence that lags behind competence is by far the more common problem, particularly among people who are actually good — and it is fixable with evidence rather than with encouragement.

The useful practice is keeping an actual written record of things you did that you did not think you could do. Not as a morale exercise, but because memory is unreliable in exactly this direction: it reads a run of successes as normal and remembers every failure clearly.