I wish I could say it gets better man. The only advice I can offer is to keep doing what keeps you functional. I cannot count the number of times I had a breakdown because I forced myself to be in a situation I could not handle being in. If stepping away for a while and playing some piano works then it works. Its better than the humiliation of a crashout. If there is something you can do while in a room of people that is soothing I would recommend doing that as well. Personally I like to fidget with pens or metal spheres, I find the weight of them to be pleasant.

Edit: I almost forgot the best advice possible. Find a safe person. Connect with the people who normally attend family gatherings but connect with them in a more 1 on 1 setting. Text works for this just fine imo. Find someone who understands your struggle and go out of your way to hang out with them at these gatherings. I got lucky that one of my cousins is autistic so we just talk at these things and it keeps most people away from us bc we look busy. Its a great system

I'm gonna go a little against what other commenters are saying here. If you like hanging out with friends and family but not in huge gatherings, then that's certainly a reasonable preference. I would suggest trying to find opportunities to spend time with people in smaller groups, and when you have to be in big groups try to take breaks being totally alone whenever you feel you're reaching your limit, not after you've passed it. Go into a room by yourself, put in foam ear plugs, and put headphones over them with music playing, dim the lights, make a calm and relaxed space for yourself to actually feel like you're alone and just chillin. After 20 minutes of that you'll have recharged your "social battery" (websearch that term if you aren't familiar with it) and then you'll be ready to socialize again for awhile.

You have people who actually want to care for you. Don't take that for granted... As others said, you may not like it now, but someday you'll regret that. Just be polite and deal with it

Maybe next time, try and come up with a fun coping mechanism. Maybe picture each person as a funny thing, ex. Pokemon. If you get distracted, you may not feel as stressed, and each time will get easier. OR make a Bingo card in advance, with all things you think will happen, that might also be a fun distraction. My 2 cents.

Yeah, you may have autism. /s

Seriously though, I was the same way when I was younger. And at 61 I’m pretty much still that way. I deal with it now better than then, but it’s still there.

I just know I’m an introvert now. Don’t think I’d heard the term back then.

You a teen, it's fine - we've all been there. I'll tell you something I wish someone had told me when I was a teen - people are trying to reach out to you, but you're actively shutting them down. Quit being a dumb edgy ass and form some memories together - it won't be long until you start missing last chances.