I don't have those extremes but I know people who do. I have had something similar but to a far lesser degree. Anytime a thought like that pops into my head I just basically say **** it... if it's real I'll either die or wake up in the ER but I'm not going to let it interfere with what little normality I have
If it alleviates your anxiety to keep going to doctors trying to find something wrong then so be it. Have you thought about when you will finally accept a diagnosis?
If this has been a problem since you were 5 and you are reasonably healthy you may want to consider therapy for hypochondria.