meditate and stay in the moment. That may sound corny, but if you do it, you'll probably see results. Once your mind starts trailing off, thinking about a possible illness, bring it back to what you're doing at that very moment. Allow yourself to only be concerned with things that are happening currently. Deal with a problem when it arises, not when it's non-existent.
Of course if you feel that this is an actual illness, such as OCD, then by all means see a doctor.