Athene
I am so sorry you had to go through that and I really congratulate you on being able to get out of that situation.
His psychiatrist was right up to a point. Some borderlines have been horribly abused, just as some with other disorders have suffered abuse. But as they have looked at it over the years, they have begun to see patterns emerging very early on in a child's development, specifically that inability to self-sooth. They think - and remember that this is psychology/psychiatry, which is hardly an exact science - that the person is born with a particular genetic predisposition - that is exacerbated by their environment. This does not mean that all borderlines were abused. They can interpret normality as abuse, given the right circumstances.
That multiple personality issue is controversial but I've seen people that I am 100% sure have completely split - compartmentalised aspects of themselves until those aspects are no longer communicating effectively with the other aspects. Borderlines don't split. They "fracture" so that they can give the appearance of MPD, but the different parts of "self" or "mind" are still communicating with one another. I know it all sounds so weird, but think of it as things like love, lust, anger, gentleness, greed, generosity etc. Most of us are made up of all those parts plus some. Those parts work together, our "better angels," for want of some sort of term, governing the darkest side of our psyche. In splitting, these aspects become more and more disconnected. In fracturing, they are still connected but not always working properly.
And after writing a couple of paragraphs, I still have to say that all of this is still conjecture because - psychiatry is so imperfect, as many with ME/CFS have learned to their dismay.
But the good news is that most borderlines begin to improve by the time they are forty or so. They learn self-soothing techniques and so are far less dependent on others to provide their sense of worth.