Do you have any stories on what it was like being with someone who had that?
I've never been with anyone like that but my foster daughter had it and I was able to observe her relationships. Because of that I read up on it a ton, looked up other people's experiences, etc.
I'd say there are four things that make it very difficult to be in a relationship with someone suffering from Borderline unless you are deeply, deeply committed. Consider this to be fully true for someone who is on the more serious end of the spectrum, but partly true for anyone with borderline:
1. They have difficulty regulating their emotions. They can go from 0 to 100 in no time at all, and once they're triggered it can sometimes be impossible to get them to back down. So when they're emotionally triggered they could literally do anything or commit any level of violence between they're so out-of-control.
2. Their minds tend to operate in the present, they have trouble stepping back and putting things in context. So if you're mad at them now, then they interpret it as you'll be mad forever. If they think you're letting them down in the moment, they interpret it as you'll let them down forever. That turns every negative interaction into an entire existential crisis, because for you it might be just a bad moment but for them it feels like their entire life is at stake.
3. Similar to #2, a lot of folk with BPD have a loose relationship with the truth, because the truth is whatever they think at the moment. They could say something completely untrue but they'll say it with the conviction that it is true, you could argue with them for an hour and you start to doubt yourself and begin to believe they're telling the truth because they're so certain about defending themselves. But that certainty comes from the way their brains operate so fully in the moment that they latch on to the position with the conviction of certainty and can't back off and think if it really happened that way.
4. A lot of folk with BPD have very very low self-esteem, and as a result they tend to overcompensate by not being willing to admit any fault (which ties in to #3). If they believe they've done something wrong, then they'll interpret that as meaning they're always wrong, they're always a bad person, and their self-esteem is already on such a knife's edge that they can't handle dealing with the perspective of being such a bad person. So they tend to distort reality and interpret things in such a way that makes them right, or makes their behavior okay, because they're so scared of the alternative.
It takes a lot of work to help someone with Borderline Personality Disorder work through their issues, and I wouldn't recommend doing it without doing a lot of reading and either being a professional or being able to talk to professionals about it. But one of the key components is basically unconditional love with boundaries - being able to continue loving the person so completely, despite what they do, so that they begin to believe they're actually loveable and that you won't give up on them when they do something wrong....but also with boundaries so they learn what kinds of behavior can be tolerated in society and what can't and start to be able to follow those limits themselves. Our daughter was an abuse victim with PTSD in addition to BPD and so she was in a tougher place then most, but I think we've seen a ton of growth in her, especially in the first 9 months. Her life isn't perfect by a long shot but she's able to do a lot of things she never was able to do before, seems to be in a reasonably healthy relationship now (we hope), and only has a little more than a year left in her college degree now.