DON'T TAKE PILLS!!
I wish I knew how to help you because you are trapped in it bad. Short of therapy or medication, can you try mindfulness? It's a form of meditation/ cognitive behavioral therapy. At the first sign of an intrusive thought, take a deep breath in and keep breathing until it passes. Make yourself aware of your fixation on your pain and SUBSTITUTE that thought with another, like I'm strong, I'm alive, this is my mind playing tricks, whatever. Maybe you will begin to notice patterns in what triggers your fear. If you google Mindfulness, you might get some idea of how to do it--- and it just might give you something else to do besides imagine you have colon cancer.