I was married to someone with BPD for 12 years. Actually still married technically because she went increasingly crazy and is basically unable to deal with anything and breaks down every time I bring up the topic of divorce. It was horrible being with her, increasingly so, it was great at first other than the sudden, horrible rages where she almost became a different person that I hated. But the rages became more and more frequent, and worse and worse. At some point I was no longer dealing with a rational person. She was unable to see anything from anyone's perspective but her own, and she refused to admit she had a problem. Whenever she went into a rage, it was my fault (sometimes for things as stupid as stacking the mixing bowls wrong). I believe that she means to be a good person, and she could also be really sweet. But when angry she was absolutely horrible. Eventually I turned to opiates and was badly addicted for 10 years, I wanted to kill myself eventually. She almost killed me once when she was angry, too. It all got increasingly bad as time went on. Finally she left (I feel like I had stockholm syndrome, I could never convince myself to leave her, I always made excuses for her behavior), and within months I had quit opiates for good and now, 4 and a half years later, my life is completely different and amazing, and I have a wonderful partner who treats me well, we haven't even had a single fight in 3 and a half years.
To this day (actually, specifically today, just an hour ago) she is still fucking with my life... staying with my ex for so long and getting married to her is literally the worst decision I ever made. Of course, that's her, I don't know your person. My ex was completely unwilling to admit she had a problem or go to therapy or anything, so it just got worse and worse because she wouldn't even admit there was a problem.
How to deal with her? Whenever possible, just concede whatever point she is trying to argue. I had to constantly swallow my pride, because if I didn't it became an abusive fight. It was hard, impossiuble really but we were living together and romantically entangled/married still, it's much easier now because it's like, whatever, I'll tell her she's right, I know the truth.
Sorry man, I know how tough it is.

Sorry she's keeping your kids from you in particular. I'm SO glad my ex and I didn't have kids.