A key heuristic for confidence seems to be “cards in your back pocket.” For example, it’s much easier to negotiate salary when you know you have other job offers. It’s much easier to confidently give a talk if you’ve done it before and know you can recover even if you get derailed. It’s much easier to be confident in social situations when you’re comfortable with your existing relationships, and aren’t desperate for new ones. Etc.
This suggests that in situations where confidence is important, you should be humble. The default seems to be to exaggerate, to try and present yourself at 120% by overstating your strengths and hiding your weaknesses. But when you present more than you have, you can’t back it up, and it shows. One idea is to play at 80% instead. Freely admit your weaknesses-or even overstate them slightly-and downplay your strengths a bit. If you must talk about yourself, give conservative versions of your accomplishments that can be easily defended.
The words that come out of your mouth may be less impressive, but the increase in your confidence will likely more than offset it, and people will be much more inclined to trust what you say.