If I can change your question to "why am I staying", here is the answer. I have only been with her for five months, I am nowhere near qualified to say I have "stayed" yet.
My uBPDgf is classic BPD in many ways, but every person is different, and from reading the posts here, I think lots of folks have it much harder than me.
First, I was lucky. I realized that there was something mentally wrong with my GF almost immediately. She is 30 years younger than me, and very beautiful, yet she was infatuated with my old fat self immediately. But that doesn't mean I completely avoided the pain of the uninformed non-BPD relationship partner. I experienced the variations in response (one day she worships you, the next day she ignores you). And I happened to be with her on a day of increasing stress which she took out on me, then got the full on rage treatment. I found this site by googling "hugely inappropriate anger".
So back to why I am staying. My GF is classic BPD. She fits every one of the criteria on the DSM except one. She also has real honest to goodness DID with at least 3 alters and list time, etc. To round it out, she is mildly schizophrenic, mostly with audio hallucinations. So why am I staying with this person? I used to ask myself that question a lot. I do less now, having decided that lightning has struck in some very unusual way, and through a lucky combination of interlocking personality strengths and weaknesses in us both, we are in love with each other. It took me quite some time to believe that she is in love with me, as she is so beautiful and sexy, and I am not. But now I have decided that we don't necessarily control or understand who we fall in love with. Over time I learned that she has only had one other stable relationship, with a man older than I, and that ended when he passed away. I have concluded that this delightfully young and attractive woman is a real challenge to live with, and that makes me understand more why she values me so highly. Her mental illness and ghetto background, along with parental abdication, let her to felony crime in her teens. She says if she had not married (that lasted less than a year) and had her daughter, she would likely still be a "menace to society" (her words). But she is absolutely incredibly focused on bringing up her daughter right, and giving her the live and care she never got.
And to add yet more challenge to our lives, we are of different races.
Ok, for some reason I felt like I needed to give you an accurate picture of her before trying to explain why I not only stay with her, but am considering marrying her.
First, to get it out of the way, her beauty matters, but it would not be nearly enough to overcome her challenges by itself.
I think for me, I stay because the good times are 75% of the time we are together, and they are VERY good times. She is very loving, and makes me very happy. Some of this I think stems from her illness. She feels every emotion very intensely, and with little filtering, and when one person in a relationship experiences bliss, it rubs off on the other. For me, this is the core of why I stay, for the good times.
Another aspect of her that I enjoy is that the child in her is much closer to the surface than most adults. She still remembers how to play, how to be silly, and is unselfconscious. We took her daughter to chucky cheese for her birthday, and at one point the kids all started dancing with Chucky. Or should I say the kids AND my girlfriend.

. Chucky thru a bunch of game tickets into the the air, and my very physically fit GF was down on the floor gathering tickets with the rest of the kids. (she gave the tickets to her daughter). The other parents were a bit appalled at her, but (remember, interlocking personality weirdnesses) it just made me laugh, and love her more. Another example: while I have been typing this, a tickle fight broke out between me, my GF, and her daughter. These are good times.
Then there are the bad times. Whoa... . When I trigger her, either by accident or when there is no choice without losing my self respect, about 3 out of 4 times I manage just to look at her as a natural phenomenon like a volcanic eruption ("geez, look at that!". But she is really good at being mean, and sometimes I just have to
remove myself from the apartment. I know leaving is supposed to be a bad thing, because it makes then feel abandoned, but sometimes I have no choice. Other times she will give me the silent treatment. Completely ignores me. That used to drive me out of the apartment almost immediately. And she stopped doing it. But she got really furious at me recently, and gave me the silent treatment for 2 days. I left, telling her as I did that she should call me when she felt like treating me nicely again.
When I leave, she starts off by sending me an hours worth of the must truly horrible, angry, hurtful text messages you can imagine. Most of them I don't reply to, a few I reply calmly to, just returning reality and love to her vitriol. After a while, she will come back to earth and apologize. She always apologizes. At least so far, she makes it clear that the relationship is very important to her.
And then comes the most difficult thing to communicate. She is making the most intense effort I have ever seen to overcome her demons - her illness - and make the relationship work. One example. Once she figured out that her tendency to ignore me sometimes bothered me, she now usually makes an elaborate effort not to do so. For most of us, decent behavior would come more or less naturally, but for her it is not natural. She has to work at it. She has to work very hard every day to try to make the relationship work. And she does, putting in more effort than I have ever seen anyone put into a relationship. It makes me feel honored, and valued. It hard to stay mad at someone who is trying that hard.
So I do know that most relationships with people who suffer from BPD crash and burn. I know that someday (heck, maybe even now) she may cheat on me, and then end the relationship. I know that happens a lot.
For me, the privilege of her company in the good times, and at least the chance of it lasting long term, is worth the risk. If it ends tomorrow, it will have been worth it.
It may be my wishful imagination, but she seems to get a tiny bit better every day. I provide her with consistent love and caring that she can count on. Maybe that helps... . Or maybe I will get a real dose of pain later. We'll see.
If a lot of that sounds like a weird love letter, well, it is.
Kevin