OP, no-one knows what's going on in their partners head. That veil exists for everyone. As far as I'm aware, my DP loves me and wants to be with me. Yes, she gets mildly frustrated when I leave my socks lying round the living room, but I'm fairly sure she's not going to leave me over that or anything else, provided I don't screw up massively.
Now I might be wrong, for all I know every time she leaves the house she's out cheating on me. But none of the evidence points to that, so I don't believe it.
As far as I can see, none of the evidence points to the fact that your DP is on the way out either. But your mindset is unhealthy, so you're grasping for evidence that he's cheating, or will cheat, so that you can end the relationship on your terms before he can hurt you. You're pushing him away, and it sounds like you're doing it to your friendships as well.
You say your bandmates aren't your friends. Why not? You spend time with them frequently, doing an activity you enjoy together. That's friendship to me. They might not be close friends, but most people only have one or two people they'd really consider close friends.
Right now, your mind is unhealthy, and it's causing you to draw wrong conclusions and make bad decisions. If you had a broken leg, you wouldn't think "Oh, I'll just walk down the shops", but because it's the part of you thats doing the thinking that's broken, you can't rely on that thinking.
Listen to the people on this thread. Talk to your DP, and talk to your friends, and then listen to their advice and let them help you. Most importantly, let them help you talk to your GP.
You can see your GP. Tell your employers that you're waiting for a call from them, and you need your phone. If that doesn't work, take a weeks annual leave and try and get an appointment. Or take a few days off work because you've got "COVID".
You have a support structure, even if you can't see it right now. We can see it, even if you can't. Trust us that it's there and use it.