Agreeing with all the calls for "Social anxiety" with the note that, though I'm not necessarily anxious, I'm super socially awkward, that I sometimes forget to say thank you. And then the time has passed and you can't really say thank you in person. If I realize I forgot to say thank you, I'll usually text them the next day or something though ("Thanks for the other night. I had fun.") which sounds a lot like the "OMG so much fun!" status was (then realizing that they might have seemed like a bit of a bore and wanting to let you know that they enjoyed themselves).

I'm getting a lot better at my thank yous, but I still have trouble with my pleases.

ALL THAT BEING SAID: It's totally acceptable if you don't want to be friends with this person anymore. Assuming they do have some sort of social anxiety disorder or something, not everyone can handle being friends with someone who's mentally ill. And that's okay, because your own happiness should be more important than anyone elses.