Social anxiety is a big problem that a lot of people have. I'll be honest with you guys it was something that I struggled with a lot in high school as well as my early days of college.
You feel anxious especially in large groups is because of what goes on in your head the moment you walk into a party you immediately think what are they whispering about? what do people think about me? Am I acting like a fool? Because of these thoughts, we start feeling anxious. We feel like everyone is watching us, waiting for us to mess up and this was basically me every party I went to. I was a nervous wreck. But that all changed for me once I started reading a book called "what everybody is saying". It's a book written by an ex-FBI agent who teaches you how to read people by looking at their body language. He teaches you things like, if someone is crossing their arms they're adopting a defensive position which means they either feel threatened or they're unhappy with the situation. Another one is oh you can tell if someone wants to leave a conversation by the direction their feet are pointed. If her feet are pointed at the door you know she wants to get out of there as soon as possible.
Once you learn this stuff and internalize these concepts you start noticing them more. You notice how people are standing, where their feet are pointed, you can tell who's nervous and who's having a good time. You can kind of guess where a person's state of mind is. What happens next is something very powerful. Instead of focusing on yourself all the time your thoughts become focused on the people around you. So the next time you're at a social gathering it's no longer "oh what are people thinking about me, oh do I look awkward". Rather it becomes "oh that's interesting it, looks like he's not having a good time"
Once your dots become focused about other people you realize that no one is really thinking about you. Most people are in their own head thinking about themselves and once you internalize this, once you've proven to yourself that this is true, something very special happens. You realize that nobody is watching you or what you do. Once you internalize this concept your social anxiety can literally disappear overnight.