Dealing with a somewhat similar situation myself. In May I moved my horses that I’ve had at home for 5 years to a boarding barn. It’s been a while since I’ve boarded (clearly) and even longer since I’ve been at a barn that has most people in a program. It can be VERY busy. I’ve only just started going out regularly due to life keeping me busy since May. It does feel a bit like I’m the outsider since this group has been together for a while (there are always new lesson kids but there are several long term established clients). The group is very welcoming and inviting, thankfully. It can be overwhelming sometimes. I’m used to it being my two horses and me and that’s it. My stall is on the end of the aisle so it’s a bit quieter which is nice. I’ve taken a few group lessons with the “ladies” who are fantastic though a generation or two older than me. I also watch the teens lessons sometimes before or after my ride and converse with the BO/ parents when I can do so without feeling like I’m taking away from the lesson. I’m one of two people in my age group at the barn which is tough too, I think. But horses are great because all ages are welcome and I still greatly enjoy socializing with the teens and the ladies.
So in short - you’re not alone, it’s normal, lessons (participating and watching) is good to build some sort of relationship, and just small conversations will help.