I feel left out when not invited to some Christmas/NewYears parties, but when I do attend the ones that I am invited to, I can't wait to leave

I saw two of my favorite coworkers outside of work together and felt a short, intense pang of rejection, but I’m honestly not that friendly at work, so what can I expect?

If you keep saying no to invitations, people will stop inviting you. See if there is something you can do to enjoy going out a bit more, that should help you say yes more often.

I never say NO when invited. If they are nice enough to ask I feel like I should go. But I get bored quickly. My fault Not their's, and look for an early escape.
- Unfortunately it's a double edged sword. If you always leave early, hosts may think you don't like coming, and stop inviting you, too. Especially if you leave before something that requires more people, like a planned game.
  For anyone who worries they'll run into this, just make sure you're really vocal about how much you appreciate the invite and all should be good in the world.

I think that's very common.

Like wanting to be different, but normal.

I'm relieved when people don't invite me to things. I never want to go and it prevents me from having to reject their offer and make things weird. I get more than enough social interaction from work itself. Any more and I burn out.

Exactly this. I get plenty of social interaction at work with patients and my coworkers.
Most hangouts with friends these days consist of virtual hangs, and group watch parties on weekends. It’s nice to be comfy at home with my cat and socialize with people on my own terms.

loneliness ≠ aloneness

Being alone isn't so bad. Now being with someone and feeling alone, that sucks.