Hey,

Due to Complex-PTSD I too have trouble with nightmares, falling asleep, and staying asleep!

Our subconscious is left to go free whilst we sleep, and so if there are any emotions or troubles from childhood (or whenever) which haven't been processed you will likely keep having these nightmares.

I would suggest seeing a Relational Therapist, they will help you truly uncover your troubles and emotions from the past, and therefore have a healthier life now and onwards (including sleep!)