I really want a friend group, but I shut down when with groups of people

I have social anxiety (diagnosed) and I think also avoidant personality disorder and selective mutism (undiagnosed). Idk if this is a common feeling, but ever since I was in elementary school I dreamt of being social and having a big group of friends. I still do, and I’m 29. I want so bad to hang out with people. I’ve had the chance; my boyfriend is extroverted and I often get to hang out with his friend group. But when I hang out with them I shut down and can’t say anything. This happens with most groups of people. And they assume I’m antisocial and just really introverted or shy. It’s better when I’m with one person alone, or talking with strangers, or even when I’m at a concert by myself. I can be more extroverted in those settings. I don’t know what it is about established friend groups that make me shut down so much. And I get really jealous of people who have that, including my boyfriend, and I’m starting to resent it because I want to just feel apart of some kind of community. The anxiety is so isolating. Is there anything that can make this better, any treatments, drugs, etc.?