Hello broccoli.brownie: I'm sorry you have had these difficult experiences. I often wonder how it is my life turned out the way it did. But, really, there is no rationale for it. It simply is what it is.

Personally, I think people transitioning into other phases of their lives, & leaving old friends behind, is probably just the way it is for most people. I guess I wouldn't really know because I've never been a person to maintain relationships.

You mentioned your father having abandoned you. Perhaps this experience has lead you to inadvertently attach yourself to others who are also more likely to leave... sort-of like a person who has been physically abused seeking relationships with people who are abusive themselves. I don't know. This really gets into areas of psychology which are beyond my knowledge base. But perhaps it's something to consider & to discuss in therapy, if you're seeing a therapist, or if you decide to see one at some future time. I wish you well...