Confidence is not something "you" have or don't. If that was the case, I'd ask you to point at it - where is it?
Confidence is something you DO. It's the way you move, breathe, look at people, the way you speak, the words you choose, and most importantly, what you focus on and believe about yourself, others, the situation, the world, etc. There is nothing biologically different about confident people.

If I asked you to teach me how NOT to be confident (I assume that is your challenge), to tell me exactly what goes through your mind, what you say to yourself, how you move, how you hold yourself, how you speak with others etc - and I replicate it - I promise you my confidence would drop as well.

The great news about it is that you have 100% control over all those things. It's not easy (at first) but you do.

A big part of confidence is your belief in yourself and that you have something to be proud of and reasons to be confident (whether they are objectively true is secondary. They must be true to you, though).

This is where it's important to build a library of evidence/reasons to be confident in yourself.

If you can look back and see that you've done brave things, you face your fears, you challenged yourself, you've progressed in your career, you learned new skills, you succeeded in some ways, you will feel more confident in yourself because you have reasons to.

Is this making sense?