I can't personally relate to all, or even most, of your problems but I think it's important to point out that you're putting a lot of pressure on yourself. While it's true that you make a lot of friends in school and that friendships tend to grow stronger over time, it doesn't mean that that's the only place you make friends.

One of my current best friends only moved to the city a few years ago and he has a solid social circle that he built pretty much from scratch. I've kept close friends from all stages of my life (pre-kindergarten, high school, university, post-school) and I continue to make friends. The hardest, but best part about making friends as an adult is you have to specifically choose the people you want to spend time with. If you really vibe with someone then it doesn't matter how long you've been friends with them for, you just want to hang out with them.

You need to take baby steps and not be so focused on the end goals that currently seem unattainable. I'm very much a conformist and people-pleaser so maybe I'm not the best person to give advice, but you need to be happy with yourself first and foremost. If you're content with the way you are and want to connect with people, then you need to be confident enough to accept that some (maybe even a lot of) people won't understand you and you need to fight through it to find the people that do. If you want to change yourself, then work towards the things you are insecure about until you feel comfortable. Or maybe find a middle ground where you try to fit in a little bit more while still being true to yourself.

Being lonely sucks, but you have to realize that the only person who can make it better is you. People aren't obligated to like you or be your friend just because you exist. You have to put yourself out there and put in an effort to build relationships with people and be strong enough to fight through the bad experiences and keep practicing and improving until something sticks. I still suck at socializing, but I'm slowly getting better at it because I just keep trying.