https://cognitiontoday.com/2018/12/the-scientific-truth-behind-fake-it-till-you-make-it/
https://psyarxiv.com/svjru/ - Sources from the article itself
Found this article today, and it argues that pretending to be confident or happy can trick your brain into actualising these things. I remember reading that physically smiling can make you happier too, because your brain associates the movement with the feeling. So, yet again a seemingly intuitive notion has scientific merit. If you have social anxiety it can help just faking confidence and over time you'll start believing it yourself.
In my experience, confidence and happiness are a bit like switches to be flicked. If it's down, it tends to spiral downward, but once the switch flicks up, your confidence starts growing exponentially. Like a Moore's Law of confidence or something.
Anyways, this really spoke to me because I've been pretending to be a big dick alpha male for years online and it's finally starting to work :)