Sounds like you have these nightmares much more frequently than me. I'm so sorry to hear that. It is definitely a very frightening and debilitating thing, not being able to even sleep without this worry and fear. My mum bought me some tea that helps you to sleep. I have had some sleeping problems because of this fear but this tea has really helped. I imagine it would calm down your brain as well as your body and this could lead to a reduction in night mares. Maybe you could try something like this? I don't remember the name of the tea, but if you go to a health food store and ask for a tea that helps sleep they will help you out. It might just fix your problem with the nightmares. It's terrible, you shouldn't have to have such scary dreams all the time. Sleep is sometimes the only escape and if it enters your dreams all the time I imagine that would be very difficult for you.
All the best, I wish you sweet dreams