I'm very attracted to her mental illness because I myself am mentally ill.

This line is so... odd to me. She's attractive because she's mentally ill?

Borderline is not a very manageable illness. It's almost impossible for someone with BPD to have anything resembling a normal relationship with someone else. It's likely that this cycle of pushing you away only to want you back is part of the BPD.

However, it's pretty much all a moot point, because she's not attracted to you. It's not really fair to declare that it's part of her illness - sometimes we just aren't attractive to others.

This situation keeps happening where I get these beautiful girls at first, they tell me I'm so talented, smart, a sweetheart - basically I have a winning personality and skills. But my looks are just average, maybe unattractive.

What do you mean, "get"? Do you date these women only for them to break up with you because they're not as attracted to you as they thought? Because that's actually a very normal thing. You also say it like this has happened to you over and over and over... but you're 21. I know you don't want to hear it, but you've barely gotten started dating at all, let alone dating adults. Most of these "beautiful women" you're lamenting were teenagers. You also were in a toxic, vague relationship with this girl, which is going to turn a lot of women off you.

I think you need some time away from dating anyone to get yourself sorted out re: dating and life in general. I'd also look to see if there's a pattern to the girls you date - if you only go after girls with serious mental issues, that might be a major source of your problem. I'm not saying to avoid people who have a mental illness, but...

You also seem really hung up on looks. She's a model, she's beautiful. I refuse to believe she's nice and loving because you describe your relationship with her as "toxic and abusive", so maybe you should also stop placing looks at the top of your list of dateable criteria.