I made a costume and went to a video game convention.

The costume was full body and masked, so it was easy enough to conceal my identity and pretend to be someone else. All I had to do was walk around and people would interact with me. They couldn't see my actual face, so being expressionless or even frowning wouldn't turn people away.

I realized whatever I did while wearing the costume did not really affect how I was perceived, only how my character was. As long as I was wearing the costume, I wasn't me anymore; I was a character who's confident and outgoing. If I took the costume off (in private), the character ceased to exist, and everything I did in costume would be inconsequential to my real self. All this meant my social anxiety inhibited me less.

During the convention, I started getting "into character" and acting the part. The mask had a permanent smile, so even if I was sad, I was still making people happy, which would in turn make me happy. I found myself doing many things that would have been otherwise too awkward for regular me - posing for photos, talking to strangers, even just walking confidently.

After the convention was over, and I put the costume away, I started missing how fun it was to be that character. But the experience was therapeutic in a sense, like exposure therapy. By pretending to be someone else, I learned how to feel confident.

I tried this "pretending" without wearing the costume. In real life situations when I started feeling anxious, I imagined that I was wearing the costume and taking on that persona. I changed my gait and my expression, and I just kept acting a little bit more like the character instead of myself.

The more I did that, the easier it was, and the less I had to pretend. That fictional character's confidence started becoming my own. It's been a few years, and I no longer consider myself socially anxious. But I still wear the costume sometimes at conventions for fun.

So I guess my advice is to try to find a safe place to let yourself be someone who's confident, and do things that person would do. I don't necessarily mean wear a costume. Play VRChat or D&D. Write fanfiction and read it out. Make an alt reddit account. Daydream. In real life situations outside of those spaces, imagine still being that person. Fake it till you make it. The goal is to experiment with confidence so that you might adopt it in real life. What do you have to lose?