My ex (20f) is diagnosed borderline personality disorder. 
She has the emotional maturity of a child and threw full blown tantrums. Usually involves screaming and crying, cussing me out, very dramatic.
She has thrown knives at me, threatened to set me on fire while waving her lit cigarette lighter around, threatened to cut off my dick, punched me in the face, laughed at every part of me, mocked me, mocked my relationship with my abusive father, threatened to kill herself while pregnant and the list goes on. 
A lot of it was all talk but I think if she got angry enough, she'd probably hurt me. Because she did throw a knife and thankfully missed. 
She's really bitchy when she's low or out of cigarettes, even me breathing too loud or eating too fast sets her off then. 

She got pregnant with my daughter way before I knew she was like this, she hid it for months. When I realized she was mentally not ok, everything pissed her off. I was walking on eggshells all the time. When she got pregnant, her moods intensified x100.
Our last huge fight, she told me she didn't care if I died and threw knives at me. It lead to us breaking up and I kicked her off of my property, she slowly moved all of her stuff out of my place and tormented me the whole time. Even though she never "officially" moved in, she was always at my place, her apartment was probably growing cobwebs. She basically lived with me and acted like we lived together.

Now that our daughter is here, every time I visit, she makes it about her and makes it miserable. When she's in a good mood, she's clingy like a leech. She had a full blown crying meltdown when I rejected sex, she wanted sex only less than 2 weeks after giving birth. We'd already been broken up and I told her we aren't going to work out and I do not want to go have sex with her whenever. She continually calls me and texts me begging me not to leave and saying that she promises she'll make an effort to get treatment but it's not anything I haven't heard before. 

It has been an up and down cycle. When she's not having a full blown bpd meltdown, she's an amazing person to be around, she is funny, jokes a lot, is the sweetest person ever, she's great until something ticks her off. Sometimes I see the good sweet side of her and want to believe that is all her, or that I can fix her but i cant.
I feel sorry for her because she has pusher her whole family away and has no one, she's a child. She has refused therapy or treatment numerous times. She doesn't want to believe that she is not ok. She hid the fact that she's bpd from me for over a year. 
I'm at the very beginning of a very long custody battle over our daughter and thankfully have a friend helping me with the legal side of things.