Is Ativan the only benzodiazepine you have tried? Xanax works best for me with occasional temazepam for sleep. I was taking pretty high dose xanax a decade ago when my ptsd was really bad and I slept 12-16 hours/day so at least when I was sleeping I wasn't having flashbacks. Fortunately I have only occasional nightmares.

Have you tried zoloft yet? It is supposed to be a first line drug for ptsd but I can't really say that it helped me.

Are you still considering firing your pdoc and trying another? Yours seems ineffective to help you.