I agree that a therapeutic relationship seems particularly well-suited to serve as a good compromise between a sense of belonging and not having to manage all the complicated aspects of actually being part of someone's life. I'm not sure how many therapists would agree, though.

I wanted something like this with my previous therapist, to shape that relationship into something that isn't quite part of either of our lives. but when we're there, we share it, we belong, and work together. I'm not saying I didn't at times wish I could be part of her actual life, but it also felt a lot safer not to, so the therapeutic boundaries seemed kind of convenient altogether. I mean In 'real life' I felt like I couldn't possibly contribute enough to ever feel her equal. Whereas in therapy, in theory, I could. In practice, there was a connection to be sure, it was quite difficult for both of us to let go, but it was some weird mutually toxic sort of closeness. I hardly ever felt anything I brought to the sessions was valuable, and it was practically impossible to talk about the relationship itself. With new therapist, so far so good - I might be lucky to have found someone who's more open to shaping the relationship based on what works, but I'm only just starting to explore possibilities in earnest, so I might be in for a very rude awakening in the near future when I trigger something that the previous more careful approach didn't set off..

This is also true in general - as much as I wish I could be part of someone's life, I feel too inadequate to actually want to try, and I'm too ashamed of my life to want to make anyone else part of it. In particular, social interaction tends to be way more exhausting than any enjoyment I might get out of it. Except for spending a limited amount of time with close friends, and chamber music. So I normally go for group activities that I enjoy, like making music, natural history walks, or nature related volunteering - where the main focus is on the activity. But none of it works all that well. Still loads better than nothing, of course.