I used to have dreams of a black reaper when my grandmother died. I thought I was OK with her dieing, but there was a part of my mind that wasn't. When I addressed my fears of mortality and uncertainty, I was able to accept them and the nightmares went away.

Perhaps you should get some professional help, as they may be able to dig out what is causing these nightmares and help you deal with them.

The only other tip I can think of offering that might help is that every situation has an opponent. It seems like a part of your mind is playing 'Devils Advocate' and is trying to convince you that you don't have control of the dream world, or perhaps even the waking world. The opponent to lack of control and uncertainty is concentration and clarity. Concentration is achieved through practice of focusing the mind on its object (like on the breath during meditation). Clarity is achieved through knowing the object thoroughly (like the dream character is a component of my mind and by definition, does not really exist and must obey my concentration). Belief is the power that will give strength to these opponents. Belief is constructed from a foundation of reason and experience.

I hope this helps in some way, and that your terrible nightmares vanish into thin air where they belong.