Confident at school, shy again at a new job? Confidence is partly local. It rebuilds faster every time.

Confidence is less a trait than a record. Where you know the people, the rules and what you are good at, you have evidence to stand on; somewhere new, none of it comes with you. That is how the same person is loud in one building and silent in another, and why it rebuilds faster each time.

Give a new place a few weeks before concluding anything about yourself. Get reliably good at one small thing there so you have something solid to stand on. The rest usually follows from that.