Ow, right in the feels. As an introvert myself, your experiences echo mine exactly - except as I'm sure you've probably found too, when you grow up and continue being introverted and declining most things, eventually your friends give up trying to drag you out to them...

And/or develop an assumption that you don't enjoy their company and are therefore not worthy of being part of their circle on those occasions you do want to hang out with them, and kind of give you the cold shoulder treatment and exclude you from communications between themselves etc.

I'm in this odd little predicament now where I ended up going to the bar with my dad, see them in there, fail to socialise particularly well (he is either with me and ends up holding up all the conversations because he's way better at it than me, or stays put and does some reading), skulk back to the corner with him, they clearly think they're second place and cut me out more and more and it sort of feeds into itself... sometimes I don't even pluck up the courage to go over because I don't want to just stand there feeling awkward and just manage a hi on the way in, busy myself being 'antisocial' on my devices with my dad and say bye on the way out, leaving them no doubt thinking I couldn't care about them less and etc etc.

Being an introverted adult is hard! People tend to expect you to have "come out of your shell" by then, and it especially confuses them if it fluctuates and some years you retreat into it a bit. As a kid you'd be loved by the kind ones, bullied by the gits and ignored by everyone else but as you get older it all sort of ends up a big hurty mish mash of social politics.

Okay, I went on a vent there :p