I had horrific nightmares for years, when my FWH and I first got together. Slowly, they tapered off hel helped by starting to recognize when I was heading into one and stoking me, whispering in my ear "you're safe," and waking me if I kept on going down into one. After DDay they came back with a vengence.
For me, it was my FWHs dedication in doing the above when we were in the same bed. If I couldn't stand him to be, he slept on a couch outside of our room and was able to hear me going into a nightmare about 1/2 of the time and react to it. On the nights that he couldn't hear me, he would comfort me in the morning, apologize specifically for putting me back there, and be very attentive to my needs bringing me coffee, breakfast, or on really bad mornings, just helping me get out of the bedroom.
When I said apologize specifically, I mean that he didn't just say "I'm sorry," or "I'm sorry for what I did." He would say, I'm sorry that my actions made the bedroom an unsafe place for you. I'm sorry that my thoughtless actions cause your nightmares. I'm sorry that you're having such horrible mental images it's my fault, I put them there into your head.
Be specific about why and what you're sorry for. Generic "I'm sorrys" get old after a while. And if you're going to try to physically comfort your BW while she's in the midst of a nightmare, be very gentle. Strokes, whispers let your presence be there but not in an abrupt way. If you startle her awake, you may frighten her even more or she may flail out in a night terror.
Hope you can use some of my suggestions. They worked for me, but I know that that doesn't necessarily mean that they'll work for Heart.