I faced this problem about a year ago. I followed the steps given below. Now I don't dream horrible events (nightmare). I am free from them and sleep happily. I would like to ask you to practice this for two weeks. I wish you may success
Step 1: You should reduce viewing seen with horrible events (films, TV etc)
Step 2: You should be able to reserve at least ten minutes before you go to the bed. You sit on your bed or chair relax and close your eyes (don’t sleep) and think followings for at least four minutes out of ten. (If you are alone you can say in low voice and slowly or sing)
May I be safe from inner and outer harm! (Inner harm: fear, worry outer harm: physical danger or external danger enemy etc)
May I be happy and peaceful!
May I be healthy and strong!
May I be able take care of myself, joyfully!
Repeat these four phrases as many times as possible (at least three times)
Step 3 : Remaining six minutes you think followings
Same as me
May all beings everywhere be safe from inner and outer harm !
May all beings everywhere be happy and peaceful!
May all beings everywhere be healthy and strong!
May all beings everywhere be filled with Loving-kindness!
Repeat these four phrases as many times as possible (at least three times)
Step 4:
Go to bed and sleep.
Ultimate objective of this process is to convince your mind all living beings should live peace and harmony with Loving-kindness.
I am 100 percent sure that you will success if you do this for two weeks. But do not stop this as you get many other benefits if you do so. (You become famous and looks younger than your age etc.)