I don't think you can suffocate in your sleep from holding your breath. I know if you are awake and hold your breath for a long time you will pass out. Then your body takes over and you breathe. My guess is that the same would happen if you hold your breath in your sleep. The headache and symptoms you describe would make sense.

I am sorry for what happened to you, and it is understandable that you would have nightmares from it. Therapy would help with the nightmares, as it would help you process those memories in a way that they don't keep popping up unexpected like they are doing now. We can tell you that what happened was not your fault, but your feelings are still real to you.