It sounds like your parents have really worn you down over the years, I can't imagine you're as horrible as you think you are. I would definitely, definitely start seeing a therapist as soon as possible, and yes, practice being interested in other people around you. Some people are good, some people are bad, most are in between, and nobody's perfect.

It can be hard to find a relationship no matter who you are. I'm relatively attractive, funny, and have a large friend group at this point, and yet I'm 26 years old and have yet to have had a serious relationship. Don't be too hard on yourself. I think you're right to focus on building friendships and then hopefully someday the right relationship will come along.

I was homeschooled for many years and was thrown into high school with no social skills whatsoever. It can take trial and error to make friends and figure people out, (i find myself questioning every interaction I'm in) but keep trying. I'm rooting for you dude. You sound really self-aware, and life's a process. You're just now starting to get more time away from your really harsh and negative parents, right? A lifetime of being told you're ugly and that your sisters matter more than you is gonna take time to get over.