Let’s start with the obvious, shall we? You need to stop dating people who hate themselves. If someone doesn’t practice self-love, you can pretty much guarantee that they won’t know how to love you.
You need to stop dating people as an experiment. “I wonder what it’s like to date someone short or depressed or Republican or Christian or vegan.” Stop. Stop. Stop. You know what it’ll be like. You’re no longer the person swimming in naïveté. You understand things now. You understand people, relationships and dynamics. Most importantly, you understand yourself. You no longer have the luxury of pretending that you don’t know any better. (And that sucks because the second you figure things out, life becomes less fun, doesn’t it?)