My therapist has been talking about parts, she briefly discussed DID but stopped because I was fighting tears, mostly I refuse to acknowledge her comments on the topic. For the first time last session, she addressed me in the plural saying things like, "I need all of you..." and "You're homework for this week is to love all of your parts". She discussed how some of my parts take on the abuser role of my father, while other parts play the defiant role against the abuser role. I'm always battling myself. I too take on a very childlike behavior and feelings/ emotions when upset. She asks me how old I feel and the answer always comes back 4 or 12--but I refuse to answer the question out loud. I've always talked to the 4 year old part so long as I remember, and can tell you exactly what she looks like, she is separate from me but lives inside my head for the most part, I've always assumed everyone had that. Like you, I stay aware, but can't really help the behaviors or feeling of being that age in the moment. I am that person.
If emotionally upset enough, I am unable to speak, selective-mutism type. I cannot talk if I want too, and the inabillity to communicate lasts long after I've calmed down or re-regulated. I almost wonder if that might be part of everything? I'm not sure, or just strong dissociation in the singular one-person sense. If pushed beyond that when emotionally upset my entire body goes catatonic, which my therapist once told me would only happen if there was something horrific that had occurred at a very young age. I don't know at all.
But yeah, I fall into that younger self, and am aware of it in the moment, but powerless to stop it. I don't go near my therapist and wouldn't let her near me during it. The 4 year old self that I've always had in my head lives under a table and is terrified to come out and play because the adults might hurt her. She wants to, but she is so beyond petrified, she's usually on the verge of tears, so she stays put. I think it's why I cower in the corner as far away from my therapist as I can when I revert back to that childlike state. I'm positive that if she came near me, there is a huge chance I would get up and run.
The idea of DID terrifies me and I refuse to approach it. I think if my therapist knew about the little girl, she would probably ask me to communicate with her, and so long as I was allowed to do so without speaking outloud and strictly with internal conversations, I could do so.