I find that the single greatest challenge to understanding all psychological disorders, is the fact that so far, there is absolutely no OBJECTIVE way to arrive at a decision of what label to use on someone.
Most disorders have more of a SPECTRUM of POSSIBLE symptoms, than they have any firm requirements. BPD certainly fits that. And because no two people with the same diagnosis/label are ever alike in all ways, a lot of people still refuse to admit that there even IS such a thing as a mental “disease.”
I am of the opinion that I have been heavily involved with at least one, and possibly two BPD people, after doing a lot of reading (and first hand observation). If nothing else, the term Borderline Personality fits them, because they can only barely be said to have a DEFINITE, even intermittently stable personality. Instead, they appear to be purely reactive to where ever they are, and whoever they are with.
The worst of the two, was not the violent one, ironically. The worst one would better be described as “always acting out her life, as though it were a scripted film.” It meant that her own life story would change, the moment something new happened, that required her response. This meant that I eventually realized that I actually knew almost nothing about her for sure, and since she seemed to be unaware of the process she used to get by in the world, I don’t even know if SHE could tell the difference between what had actually happened to her, and what she had made up out of fantasy to fill the gaps, or to make whatever was going on in the moment, more fun.
I notice that most of the symptom lists that people gave above, weren’t what I would recommend using for the purposes of a thread question like this. They were more lists of “what goes wrong for the victims of the disorder” than they were lists of WHY THE DISORDER WAS THEORIZED. I think it’s actually more difficult to figure out what something like BPD means, based on what goes wrong for everyone.
It’s also, I think, a little bit of an oversimplification to say that the difference between someone with an official disorder and someone who is “just a royal jerk” is that the one with the “illness” can’t help themselves. Some people with real disorders, actually have the problem that they CAN help themselves. The woman I described above, was all about “helping herself” get along, in fact, most of her worst behaviors were the direct result of her helping herself. She just didn’t have a stable inner starting point (i.e. a true independent personality) from which to pivot.