Think of it in this context first - Introjection is sort of the opposite of projection. With psychological projection, you take aspects of your inner world and project it unto reality/the outer world. With introjection, you take from the outer world and inject it into your internal reality/experience. They are the inverse of one another.

An introject, as I used the term, is a concept from Object Relations theory, which I think originated from Melanie Klein. It's internalizing the 'essence' of your parent (usually the mother). People who had 'good enough' parents internalize good aspects of their caregiver, which helps their sense of self develop. For example, internalizing your mother comforting you would provide you with the ability to self-soothe as an adult. If your mother was scary, abuse, or even ill so neglected you, you can grow up without the ability to self-soothe. Like others here, I never developed the ability to self-soothe on my own....

It's said that infants--who don't have emotional or cognitive maturity developed yet-- can't distinguish between the inner and outer world. For example, they wouldn't be able to tell the difference from having a headache (that came from within/internally) and an object falling on its head (from external source). Same with significant people around it. Because the baby can't distinguish what emotions are hers and the others yet (there is no psychological boundary/sense of self developed yet), she can internalize the emotions and experiences of the caregiver/mother as being her own. And I internalized some scary things from my mother...e.g., her wanting me to die, sadistic, abusive, etc.

I guess what makes it dissociative is that it feels like a separate person from myself-when i'm aware of it, it really feels like I'm possessed by it-held hostage-and my real self has no control of me. Back then, it literally was separate as it was my mother, so it makes sense to me that it feels this way.

Psychoanalysis - Melanie Klein And Object Relations - Infant, World, Objects, and Anxieties - JRank Articles