No, nothing else besides DID/OSDD-1/P-DID can cause identity alteration, but other disorders do have identity diffusion as a symptom, such as BPD and ASD (Autism).
Identity diffusion is feeling alien, different, or like you don't have any real sense of self (like the you in front isn't a person. I particularly experienced it as "everyone is different from me, so I can't have any of these disorders", but someone else can experience it like "I feel there's no me in comparison to others, I have no identity compared to them." and just sort of blend themselves into other people.
I don't experience possessive fronts or blackouts, but I realized with time and much paying attention that I do experience dissociative amnesia in some forms, while not in others. There are multiple forms of dissociative amnesia, I've made other posts about it, but while textbook systems experience continuous and generalized amnesia, not all systems do. Not remembering the past, having black or empty spaces in past memories, feeling the past week or year is blurry, not being able to remember childhood or specific events in life, those all count as dissociative amnesia.
Our brains will often fill in the gaps of memories, making us feel like we do remember well, when we actually don't. I recommend writing a journal and before you look back at last week's journal entry, try to remember what you wrote in there, see if there's anything you remember, and then actually look back. It's surprising how much we forget and don't even realize it.
Trauma that causes these disorders are not inherently severe. It's actually prolonged or repeated trauma that causes DID and OSDD-1. So little t trauma counts, such as bullying, moving around a lot, divorce, emotional neglect (this is a very common cause actually), or it could be multiple things. It doesn't have to be severe or intense or seem out of the blue. It's that and an inconsistent/disorganized attachment to the primary caregiver. So if you felt in your child brain that you couldn't rely on your parent, or person who took care of you, then you would have likely developed dissociative coping mechanisms that led to being a system.
DPDR exists within and as a part of DID/OSDD-1, it's like a pyramid of symptoms, and at the top has the most symptoms, and down at the bottom has the least symptoms. DID has the most, so DPDR and C-PTSD exist within DID, along with dissociative amnesia and other prominent symptoms. It's more that the symptoms are chronic and constant, more than anything else. So if you catch yourself dissociating everyday, depersonalizing everytime you look in the mirror, feeling detached from friends and loved ones that you felt like you knew, and now you don't. Those are all common within systemhood and this disorder.
And yeah, the fact that things feel jarring or inconsistent are a major part of this disorder itself. I would often catch myself sobering up quite quickly after bawling my eyes out, for no reason at all. It would just end up that way. You don't gradually change because you repress those aspects of yourself, aka dissociate away from them after they front. It can feel stressful because it is. And then you dissociate from that stress. That's pretty normal with this disorder. So there is no need to be afraid of it.
I hope all this helps you. I promise most of it is just the dissociation, as it is a dissociative disorder. You feel weird and awkward and unnatural because we don't allow ourselves to grieve, cope or become aware in any sort of way. Repression is just about what happens every single time. You don't realize it because that's how this works.
