Look up derealization and how to overcome it as your description matches the definition exactly.
Edit: Also the health concerns are paranoid delusions (sounds bad, but the words just have stigma behind them). Just remind yourself of that whenever it happens.
Try keeping a rubber band on your wrist and whenever you feel things are not real anymore snap it on your wrist and let the physical feeling remind you that you are real, the world is real, and you are not dead and instead very much alive. Remember "I think therefore I am," and feelings are a product of thought so the same could be said as "I feel therefore I am."
Good luck on overcoming the issue. It's not as uncommon as you may think and can get better. It just takes therapy and that does not just mean a psychologist who listens and suggests or psychiatrist to prescribe meds, but any activity that is therapeutic and reminds you of the beauty of life as nothing so beautiful can be really dead.