It's a cultural phenomenon rather than a mental one. One of the most famous examples was later proven to be a fraud (See Sybil). These patients improve once they are no longer seeing their therapists, and it is thought that these therapists are encouraging these patients to find and maintain these new personalities.
I can't speak to whether MPD is legit or not, but I have known people, known them very well, who became totally different people when they were drinking. Incredibly different. Like someone you wish you'd never met. Sober? A wonderful guy. Very trippy.
My mother was diagnosed as bipolar some time ago. (though it went undetected for about 44 years and through a very successful career in the military) She has quite a bit of other medical issues, but it's a daily struggle really. I was diagnosed as being borderline when I was in high school, though I've been told more recently this isn't the case. I definitely feel it some days. Take care of yourself, it's always difficult to fathom in the moment but, I assure you you'll be missed if you're gone. You may even find you regret what others went through in your absence. It's a gift and a curse like you said. I'm a firm believer that each person has their own individual struggle, it can define you or you can make it a footnote in your legacy. Even this.
