Severe bpd is basically caused by a person who is on the osdd spectrum but doesn't know it, and has enough symptoms of BPD to get a diagnosis; but the symptoms of bpd are just that.. byproducts.
so the different parts of a severe bpd person all have different triggers and emotions and internal traumas, but they know they are all part of the same person, and they all have various pieces of a full identity as a person, but there is so much internal disagreement that they can't function in life and they constantly feel like they have no identity. With enough therapy they can all work together and sort of be normal. But therapy doesn't actually heal the trauma that separated them, which is why they never get better and kill themselves at a very high rate. -and because these people are so difficult to work with, the trauma rarely gets uncovered. also, they don't want the trauma uncovered because to do so is too personal, and it conflicts with their identity as a person. (which isn't fully formed and needs to be changed, but they often don't want to)
The osdd reality of a bpd person's mind is why can go from happy to sad to depressed, authentically, in the space of 2 minutes, they are switching, not logically but emotionally, and they don't know it. (some do)
When they split, connections break down and they become dangerous, and appear to be more like a skitzophrenic. Which is why BPD was originally borderline schizophrenia disorder. shortened to just "borderline" . basically 100 years ago they wouldn't get a schizophrenia diagnosis because the psychologists couldn't figure out if a BPD person actually believes their delusions.
For example, I got some insight into this when my wife a few days ago started accusing me of abandoning her in her apartment 5 days a week. (I would visit on weekends, not yet married.) this delusion, is true.. from the perspective of the 4 yr old part of her mind which is disconnected from reality in various ways. The 4 yr old part of her mind doesn't fully understand that she is part of the same mind as the adult part that moved out of her parents house and got a job lol. -the 4 yr old doesn't have access to the logical portion of the mind in order to process the emotions she is feeling. why? because of trama.. at age 4. we have access to that trama but she isn't ready to deal with it.
now that 4 yr old part of the mind.. is actually in charge of the whole body, and mind. and.. turns out.. essentially created the other alters, as early as age 5. 4 years ago she could throw temper tantrums like a 4 yr old, and not remember it, or later insist she didn't have an episode.. because the adult part of her mind was turned off while the 4 yr old got pissed and doesn't know what she said or did.
Anyhow she easily met all 9 diagnostic criteria from age 13 to age 23, and was in denial about it because it can't be fixed.
well in a sense i fixed it, 3 years ago triggered a split in her mind and she became self aware internally in her mind that she was actually more like DID. she had no prior knowledge of this and later said "i didn't know we were separate people, i just thought those other parts of my mind were places i went to get memories"
turns out the 4 yr old was in charge the whole time.