Why do I continue to let toxic people back into my life?

It's a bit of a long story. It all started 8 months ago, when I started dated this girl. She was a few years younger than me, that should have been a red flag but I went with it. She pulled me in real fast. I fell for her. She said she loved me and the whole thing. but then she said we couldn't be together any more out of the blue, she said she missed her boyfriend, I think well she doesn't know what she wants and leaves it at that. But no less than a month later she is texting me back telling me she misses talking to me. I get pissed at her. Stop talking to her for a few months and think it is over.

Well a few weeks ago , she texts me again saying she just wants to talk, she feels alone and no one understands her, so I make the biggest mistake of this season, text her back. at first I just wanted to be her friend I knew her life is difficult, she has an overbearing mother, and her father abused her. So I agree to be there for her. But quickly the conversations begin to get inappropriate my old feelings resurface. She says things that make me feel happy again, She wants to be with me again and blah blah blah, my brain is telling me no but my stupid heart was like why not?

However she is still with her boyfriend, she asks me if they should break up, I make another mistake and tell her it is up to her, but she must tell him.
I should have known she didn't love me if she wasn't willing to break up with him. I go with it because I want to believe things can be better, I used her as an escape from my home life, because I am struggling with unemployment and debt and all that shit and I can't leave my house because i have no money.

Anyway so here is where the shit hits the fan. She takes me to her place and we have sex, which for me is hard because so many emotions are going on, I really love this girl at this point, but after she get's off right in the middle of it she says we cannot do this, turn 180 in the other direction. within the span of one hr goes from being intimate with me to not even wanting to touch me. It fucks with my head and my heart to the point where I basically can't move. then she tells me she is sorry, she has borderline personality disorder.

So I was trying to process what has just happened.I didn't have anywhere to go at that point because it was too late to go home. So I just laid under the blanket in fetal position the rest of the night trying to make sense of why I am such a failure at this.

So now I just feel terrible, I have no one to go to and I wonder if I will ever be able to have an intimate relationship because this damaged me so much. I can't talk to my family because they judge me. They believe all my problems will just float away if I stop dating women.( bi girls you all know that aint true) So am in a dark place right now. Wondering why I keep letting toxic people in my life, trying to reign in that power for myself. thanks for listening.