How come i keep thinking i have a serious, deadly illness? how can i get myself to stop thinking like that?

Two part plan. Get a good checkup with your doc. While there ask for the name of a local life coach/therapist who can teach you stress management skills. Health focus phobias/ pre-occupation are often a conscious cover up for an unresolved sub-conscious deeper issue. While you learn stress management you may learn the deeper issue and how to resolve it.

It is not feasible to provide an opinion without additional history, physical examination may be some tests. It would be prudent to see your doctor to see if you may be depressed. Depression is treatable. Counter-intuitive as it may seem, exercise may help. Start with low intensity exercise such as walking and gradually increase the intensity as tolerated. Drink more water. Wish you good health!