"Self diagnosing" can be dangerous, and you should probably seek out some professional guidance as well - possibly two or three to see if there's any consensus.

I want to say I felt the same during the highschool->university transition. I think I did have similar attitudes for several years. One of the only things I learned during that period is that self-pity is not attractive or desirable on any level - eventually it wasn't even attractive to myself.

You may not be destined to have a lot of friends outside of class or work, and that's 100% fine. I've never had many friends, but I do work at having deeper friendships with the ones I do have, and I've found that's something that can only come with a mixture of time and experience. As we've grown up and apart, my few friends and I stay in touch, and it's been great. I'd rather have these few strong friendships than a bunch of the casual "hi-hello" kind.

You made all these "friends" in HS simply because of shared geography - everyone was forced to be in school, so there wasn't much of a choice. It's natural that most of those 'friendships' won't survive. Don't dwell on the fact most of them weren't solid or strong in the first place. Even if many of them appeared strong, solid and "for life", trust me, 99% of them aren't.

You may want to consider taking some acting lessons and/or public speaking lessons - joining a local toastmasters group, for example. If your public speaking is good, you'll naturally have a point of conversation with people, and that will naturally lead to many other tangents, yet in a controlled space.

One of the things I've learned over the years is that many people lack a self-confidence, or at least feel awkward in social situations. Not everyone, of course, but many more than I'd guessed. I'd assumed everyone else already had it 'figured out', and that assumption has led to some bad decisions over the years (based on assuming people were intentionally excluding me, for example). Be assertive/proactive, even when you don't feel it (that's where the acting lessons come in) and you'll be regarded as such - people judge you by the behaviours you exhibit, not what you think inside your head.

I consider myself an introvert - scored highly I on Meyers-Briggs for years - but most people who know me might consider me an extrovert these days. I may even have changed some over 20 years on Meyers-Briggs, but I still consider many of the core I traits to be mine. However, I've learned to be able to be 'on' socially, which has led to some good friendships based on professional interests, and some of those are leading to stronger friendships outside the original shared interests. Only time will tell for certain, but I'm enjoying seeing where they lead.

I also realize I'm now talking past your original point a little bit by not addressing the 'friends' thing as much as just 'being in public'. Being in public and groups of shared interests will lead to some friendships forming, but few will form if you're a wallflower who never talks to anyone.

Trust me, you can change over time, but you need to adjust how you think of other people, and you need to be forceful in getting what you want. If you see someone you think you'd like to know better, ask them to grab a cup of coffee, or instigate that with a few people from work or a local club. Meeting outside the regular group/work setting can let you develop/craft/express more of your own public persona without violating any of the norms of the original group. You can see how people react, and make changes accordingly. Lather, rinse, repeat.

Lastly, don't work hard at building meaningful relationships. Well... don't ignore them, but the most meaningful friendships I have are ones I don't have to 'work' at so much, because the other person is just as interested in keeping it going. My deepest friendships now started off very casually, at work/school, and in one case we really didn't even like each other at first. We each assumed too much about the other.

"Work" at friendships - yes - you need to sacrifice your own time for the other person, but if they're never giving back to you, it's not a friendship, you're being used, so get out.

You're young. You can change your life and attitudes and behaviours to be whatever you want. Don't waste this time.