My sympathies.
tldr; You (like many others) are having an existential crisis. The facts that previously underpinned your reality no longer seem true. Your challenge is to find some way to live authentically. What you've identified, a sense of belonging, is crucial part of living authentically for many people.
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I'm 35, and though I have a growing family, I am also nostalgic for that time in my mid 20s where I had less money, but intimate friends were close at hand. Recently, I've been having a feeling that something is "over".
The best encapsulation of this feeling I've read is from the Witcher short story "The Bounds of Reason". "... a wish, as desire, a yearning. Faith that there are no limits to possibility"
The ignorance of youth is really ignorance of an unknown future stretches out ahead. As I've aged the limits of possibility made themselves apparent. That faith in limitlessness was broken, and that break caused an existential crisis.
The single piece of advice I have is to grapple directly with the nature of the human condition. There are lots of ways people deal with this: finding a purpose, finding a community (like a real one, not one based on a comercial product), finding god. Put simply, the
I've seen a lot of nice ideas: volunteering, relocating, exposing yourself to different kinds of people, finding projects, therapy. I've found those helpful. But that might not be enough.
A crisis is also an opportunity. This is a chance to reevaluate what your life means to you, and what you want to do with the time you have.
I wish you the best.