If the nightmares you're referring to are flashbacks due to past trauma, they are a whole different kind of thing than regular dreams. In PTSD, memories aren't processed in the same way and the associated nightmares may cause you to feel as if you are reliving past trauma. In my studies, I did also read about Prazosin and how this can help with flashbacks and nightmares.

I hope your nightmares lessen, wotchermuggle.

Take care.