Am I being selfish for wanting to join my friends?

I wouldn't say selfish. It's hard to be miss out on something you want to do. But that's adulting. It honestly is just how the cookie crumbles sometimes. We can't except people, even family or friends to change their lives for us. That's just not fair to them or always logistically feasible. You can't please every right. Now as for feeling bad about it, it can certainly feed the "bad guy" Self doubt, depression I totally get it. It feels horrible to think that you are intentionally being excluded. Is this something that can be repeated and you can invite two or 49 friends too?

I don't think you're selfish at all. I think it's important to put yourself first sometimes. However, it's not always possible for things to work out perfectly for everyone, every time. I totally understand where you're coming from; I have no friends because I've pushed them away before they could inevitably push me away. I am constantly working on figuring out what it is about myself that I think other people don't like/hate/find annoying, because more than likely, it's just something you don't like about yourself, and you're projecting that onto them. It helps me to write out how I'm feeling and what was happening when I started feeling that way, so I can find patterns to discuss with my therapist.

Did you express interest before or after it was scheduled. If after, that's on you because you had to work. If before, they could've worked harder to try to make it a day you could all go.

I do this all the time. I find it super hard to make close friends or romantic relationships because I’m always anticipating that they will figure out “who I really am” or get annoyed by me and leave. It gets to the point where I tell myself my friends are lying when they tell me “it’s ok” and they secretly hate me OR I start to dislike them for stupid reasons because it creates distance, and getting close to people is scary.

I understand that feeling all too well my own family do this to me