Confidence is not the requirement for starting. It is the reward for having started.

Waiting to feel confident before you begin is waiting for a reward before you have done the work. Confidence does not precede the action. It follows it. The person who looks certain in a room did not start out that way. They accumulated enough repetitions that uncertainty stopped being paralyzing. You cannot borrow confidence in advance. You can only earn it on the other side of the thing you were afraid to do.