What you are describing scares me a lot. The way I cope with this is that I think no matter how I die ("natural" causes, N, or SN), I will experience different levels of consciousness as I die over minutes to hours. Even if I have N at my disposal (which I don't), in my mind I could be in a really anxious/negative state, and that might cause me to have very distressing/uncomfortable/painful experiences while unconscious, just like how physical pain from any method (or from an illness like cancer) could trigger the same.
Having experienced different levels of consciousness in my past, I know that time stops existing at a certain point (just like it sometimes stops existing in dreams), so we can 'feel' a state persist for a very long period of time even if it's just a few seconds. What I take from this is that when I get ready to CTB, I need to make sure I'm in a calm, peaceful state of mind. I also need to learn to deal with pain (physical and emotional) so that I can maintain this peace of mind as my body dies. In a way, I'm "lucky" that I have a chronic illness that lets me experience very significant physical pain occasionally, and recently I've used those opportunities to sit with the pain and learn to maintain a peaceful state of mind while I'm letting it run its course. I think it's good "practice" for CTB or "natural" death.