How does one forgive themselves for all their past mistakes,

I suppose by learning from your mistakes and atoning, and by realizing that if you can do that, and actually let this make your values grow stronger, you're probably being a better person than most people are.

I really feel this though. I led on my first bf cause he was in love with me, without having feelings for him. Eventually I dumped him cause I knew I was doing wrong. broke his heart, I guess. Since then I've felt like I would never want to do that to anyone so I'm very particular about who I get involved with, but what I did to him keeps happening to me... It's poetic justice, but if I don't stay strong and continue to try to be the sort of person I'd want to associate with, I think my chances of eventually being with others with my own values would go way down.

I genocided an entire race but I forgave myself for it

How do you cope when your self hatred is unfounded and irrational but it persists anyway?