My friends didn't invite me to an event, and I feel left out, but I also feel selfish for feeling this way.
Really, why do you think you're having mixed feelings about it?
It Happened a lot to me. For example this week I had a dinner with a group of people and no one pinged me to join them, even some of my friends who had been on a month break from social media.
You should let them have some space. It's not good to do things that make you seem selfish.
I'm a outgoing person. I need to be with people.
Sometimes our friends just need some space from us, especially if we go out all the time. It hurts but it's understandable. If they, do it for an extended period of time though you likely aren't being selfish and something is actually wrong.