wow Howslans I feel like I haven't seen you here in forever. Hope you're doing alright.
There is no such thing as fucking BPD or any other ‘personality disorder’ one might care to mention. It’s made up pop psychology bullshit designed to a) allow pharma to sell you drugs you don’t need and b) give shitty people an easy excuse for their shitty behaviour.
This is the tricky thing about psychology as compared to other, more "scientific" fields of medicine. To begin with everything is in the head, and there's not even anything physical that can be seen in the nervous structure or lobes of the brain--to my current knowledge--that would provide hard evidence for the majority of personality disorders. If somebody has cancer, you can see the tumor, and you can surgically remove it. If somebody has a more "concrete" mental illness like schizophrenia, there are very particular symptoms that can't be easily ascribed to other complications, and I believe there is a visual tell of it in the sufferers' brains (for schizophrenics specifically, there is a change in the size of certain cortices relative to non-sufferers).
In general psychology is just such a new field, and concretely learning more about it--again, since it's all in the head--is almost impossible. It also doesn't help that modern psychology's founding fathers were also idiots, charlatans, protejectioners, and/or raging misogynists. Women's health in general, mental or otherwise, is also so extremely poorly researched and prone to misinterpretation, which is why I think some mental illnesses are so much more strongly associated with one sex than another. There's also the fact that what is and considered a mental disorder, or a symptom of one, seems to change just based on the whims and opinions of whoever's in charge. (Consider the removal of gender dysphoria from the DSM5 for no apparent reason other than troon lobbying.)
I don't normally like to do the appeal-to-history thing, since it's not always applicable, but it's helpful sometimes in determining whether something is real or a convenient modern invention. Certain mental disorders like bipolar have been described--if not with modern language--of certain historical figures such as John Adams. But what we today call personality disorders don't seem to make an appearance in history very often; nobody called the bloodthirstiest Roman emperors "borderline", we just called them bloodthirsty, apathetic, not fit to rule, in some cases traumatized, et cetera. This is especially interesting when you consider that diagnoses for these illnesses seem to get handed out left and right these days. If every fifth person today is considered "mentally ill", how could it be that only a small percentage of people before 1900 were?
Another way to determine the legitimacy of something: follow the profit. You just know that pychologists and whoever manufactures psychiatric drugs are all making a fortune off of this. And it's also disturbing how quickly people will hide behind a mental illness--something supposedly innate, genetic, uncurable--rather than admitting to a normal human flaw--something you have to work on and fix. I have an acquaintance that thinks she's "neurodiverse" just because she has dogshit time management skills (sorry, "executive dysfunction").
Personally, I do think there's some line you can cross between "horrible person" and "actually wrong in the head somewhere", but I just have no idea where that threshold actually is.