Dissociation in an effective childhood coping method to survive overwhelming and/or traumatic experiences, but it doesn't end there. As an adult, very often, the dissociated parts can be a problem--especially, but not only--if the person doesn't realize they are dissociated. Even when the adult is aware of their dissociated parts, they can still cause confusion, disruption, or trouble in the person's life.
Dissociated parts--often called alter personalities--cannot just leave on their own. They cannot be wished or prayed away, and they cannot be cast out, because they are not demons. Years ago, I heard about a friend whose dissociated parts had all integrated--meaning they no longer existed--and then later they all came back. True integration cannot take place as long as those parts carry some part of the overwhelming experience of the child. And, once ministered to by Jesus and He says it's time for them to integrate, that integration will be lasting.