Relevant details first. My mother has BPD. It didn't directly affect me until her last boyfriend (not my father) finally left for good after 10 years together and she was WAY past the wall. Me experiencing the
full effect of a BPD woman happened in a relatively short window of about a year, before I broke free which was around the time I joined this forum. While there was nothing romantic or sexual about this relationship, I would argue it was worse to get out of compared to just a normal relationship, it's a bit harder to dump a parent... I also have a bunch of observations about how she has acted with previous boyfriends. So while I don't have direct experience with BPD girlfriend, I certainly have seen WAY more than I wish I did...
They will typically be very sweet and seductive at first, you might get the feeling that it's too easy or too good to be true. Trust your gut on that one, it probably is. It's their "hook", they'll get you addicted to that sweetness. Their demands will almost imperceptibly gradually increase, their typical victims will comply because they want her back to giving that sweetness that hooked them. The problem is that the demands never ends, she will demand ever more and require you to sacrifice ever more of yourself. If there's any spine left in you then you'll wake up at some point and realize she's insatiable, an emotional black hole. She will take far more than she gives back. If you don't wake up you'll literally lose yourself and become a slave to her desires, you'll be fvcked (and not necessarily even literally).
Common themes are constantly broken promises, bad excuses, gaslighting, general aggression and the range of emotional manipulation may go all the way up to suicide threats, of course blaming you for it. Abandonment issues are often central to their condition, if they're being a b!tch and you appear convincingly about to leave, then they might do an EXTREME 180 in order to pull you back with sweetness. What you'll see is a bunch of groveling and excuses that will never feel genuine enough, trust me, it won't last long at all before they prove they learned absolutely nothing.
By the way, if she lost interest & began treating you like garbage, that doesn't mean she is BDP... She just lost interest, even the nicest of girl will treat you like crap, once she loses interest.
This is dead wrong. If a BPD "loses interest" she literally can't help herself from bothering you every way she can, unprovoked. They don't really lose interest, they're emotional black holes, they NEED you. She WILL want you back under her control, but if sweetness fails she will become extremely toxic and manipulative.
The nicest of girls will genuinely only want you out, they'll just ignore you, not actively retaliate and treat you like crap. They might treat you like crap if you continue bothering them in an attempt to get her back though, as I said, they just want you gone. Going out of ones way to treat someone like crap proves continued interest, why put in all that effort if they don't care?
This post got long enough, but I'm open for more questions. Btw, you managed to type BPD wrong every single time in your first post, you wrote "BDP".