i would go through this each time I discovered an alter had integrated with me. I used to be so afraid the dream and feelings with it meant i was splitting into alters again. my treatment providers explained to me dreams are a mixture of our emotions, experiences, fears, hopes, wishes and even have elements of unreality.
In my situation dreaming something so triggering that in the dream I was dissociating\switching into alters was a mixture of my happiness that i was one whole person again, my fears of living life as a non DID person now, my brain knowing what my dissociation symptoms are and knowing who some of my alters were.

my psychiatrist told me to do something for him. for a week right before bed think about how it would feel to run my finger nails on a chalk board (something I remember fondly doing as a child) remember and think about it from the perspective of all my senses, as if I was doing it right now.

the result i was totally amazed. I dreamed I was in a classroom running my fingernails on a chalk board. I woke up in total shock that it felt so real as if I was really doing it.

so then I tried it with winning the lottery. lol amazingly i woke up and was totally shocked that there wasnt a million bucks in my bank account. thats how real that dream felt.

when I told my psychiatrist about this he smiled then said now i will remind you of something. with DID reality testing remains intact. the fact that i could not tell if it was real or not said to my treatment provider it wasnt mental disorder related and just normal. boy did i sigh with relief.

since then i have done this with many different things \ topics. treatment providers in my location call this the therapy technique called lucid dreaming (knowing you are dreaming and reshaping \ guiding ones dreaming based on desired outcome, and powers of suggestion to combat nightmares and anxiety)

the brain is an amazing thing. it can bring our emotions, experiences, fears, hopes and wishes to the point of feeling so real that its hard to tell reality from fiction.