You need some clarity and he needs the assistance of a mental health professional. Does he legitimately have dissociative personality disorder (DID) or is he just acting out because he is mentally unstable? I cannot determine this information based on your short letter.
This situation is concerning for two reasons. One reason is that he may in fact have a DID diagnosis and be unaware and unable to control how he behaves when one of his “other” personalities emerge. If this is the case, and he loses control when this situation occurs, this may put you and he at risk.
My second concern is that he doesn’t have DID but is just mentally unstable. Based on your letter, his personality changes rapidly and unexpectedly. And when it does change, you become the target of his abuse. This is not only unfair to you but the fact that his personality changes so dramatically and unpredictably puts you in danger of a physical attack.
Under each scenario, you are targeted, blamed and as you have said, emotionally abused.
Please know that people with DID are not known to be dangerous. I only mention dangerous as part of this response because of the unpredictability of his personality shift and the apparent inability he displays to control his actions when his personality changes. His personality shifts seem abrupt and often when these incidents occur, as you have said, he emotionally abuses you. I would offer this same warning to anyone who is dealing with an unstable person who is acting irrationally.
For your own protection, ask him to seek professional help, and at least a psychiatric evaluation. If he refuses help and his out of control, emotionally abusive behavior continues you may have to strongly consider ending the relationship. I hope this helps.