"It goes way deeper? But what would you call it, what is the issue? It feels like I was just put in school to socialise (where it was just "sink or swim") and the rest of the time it didn't matter to anyone if I was isolated or alone. "

What happened when you were in school? Did you sink or swim? Did you make friends?

You keep saying "it doesn't matter to anyone if you are alone", that's a strange way of thinking about it. No, it generally doesn't matter to anyone if you are alone. It only matters to you. Others are thinking about themselves, just as you are.

How did you come to live with your flat mates? Are they people you would like to be friends with? Are they just people you have a polite acquaintance with in order to share a flat?

What kind of people are you attracted to as friends?