The thing about the human brain is that most of what you feel in any situation actually happens after the event itself. If you get shot, it takes a minute for your brain to process the fact that you just got shot, and it should be telling you to feel pain. for that reason I don't fear death. I don't believe we exist after we die, and if I'm correct my death means nothing to me. If I can't exist to regret or "feel" my death, whatever I'm feeling before I die will simply end. No fuss, no pain, no worry, just nothing. Which is fine by me.
As for something like cancer or other diseases/injuries, yes the possibility is very real but knowing that I feel it would only double my losses to worry about it right up until it actually happens. If I'm going to get cancer, so be it. I'll with that when it happens, I'll not waste time worrying about it.