jan i have 2 foolproof methods you could try whilst experiancing a nightmare to break the cycle,

method 1 is imajine you are in a libary chose any book and try to read it, it scrambles the images in your brain has you cannot read whilst asleep, dont believe me try it.

and method 2 is to imagine a clock or watch and try and tell the time, the face of the clock will either be frozen or running backwards, it also scrambles the thought process and tells your body you are dreaming/ haveing a nightmare taking you safely out of the nightmare situation.