One of the major reasons I don't really like getting too deep into what caused my PTSD in therapy is because I am afraid it would be re-traumatizing and I don't think my therapist would know how to deal with that and it likely would not be a very safe situation for either. Perhaps your therapist just doesn't quite have the qualifications to help you work through those things in a safe way that doesn't leave you feeling overwhelmed and suicidal when you leave the office.