My T taught me a lot about this. We build elaborate defenses for trauma our mind just says NO CANT HANDLE, but repressing the trauma takes a lot of energy and vitality. It doesn't just disappear into the unconscious at no cost. However, when defenses get dismantled, especially disassociative ones, we can feel reattacked/ retraumatized bc the memory slices are unmetabolized/ undigested. It is like pockets of the real experience are there, and it is pretty terrifying if you're not ready for it, and even if you are. Nightmares and intrusive memory fragments are the brain's way of saying HEY don't let THAT happen again. There are several theories about how best to process these images and memories, but it takes so much courage and endurance. It is definitely true that mindfulness can help get distance- yes, there are scary memories filling my head, but it is an activity of my brain, not me, and I can watch from a distance like fireworks in the sky five miles away. Prolonged exposure therapy is another approach, EMDR is one. It is really hard.