Well, in my own experience as I struggle with complex PTSD myself, is I can experience flashbacks or things from my history that surface in an "intrusive" way. I don't "make" these experiences happen either, they are "intrusive". And trying to talk about my history in therapy often did open a door so to speak where often "more" would come forward too. Yes, I would try to find ways to ground myself however sometimes something would come forward and all I could do was to wait until it ran it's course and dissipated. I am just wondering if your therapist wanting you to talk about (ruminate about) your history can be like poking at something that brings things to the surface and next thing you know you are revisiting something from your past.