One theory about nightmares is that they serve as exposure therapy for stressors. If your nightmares are too extreme, maybe you could set aside during the day to enter a calmer environment and try to review them without getting overwhelmed. It might desensitize you a little bit and make them less severe.
I haven't had severe nightmares since I was a preteen, but when I did, they tended to be pretty stressful (being kidnapped, being abandoned, my friends commiting suicide in front of me, etc). I'm not sure exactly why they stopped, but I eventually 'burnt out', and I just stopped really caring about them.
Can't say I turned out great, but I can say I don't have nightmares anymore.