If anyone has the antidote to health anxiety they really should win the Nobel peace prize
However these are a couple of things that have helped me:
Remembering that most horrible illnesses are not an acute emergency and that waiting a couple of weeks at least would be unlikely to change the outcome. Therefore when a new symptom occurs, decide that you are not going to Google or make any medical appointments about it for a set time frame eg 2 weeks.
This can give you some breathing space and often the thing will either go away or become clear that it is something benign.
Instead of going immediately to the worst case scenario ie headache= brain tumor, remind yourself that statistically the vast majority of people with whatever symptom you have will have a much less serious issue than the worst thing it could be.
Exercise. This helps release endorphins and positive neurotransmitters, and increases confidence in your health and your body. Logically if you can complete a workout, you are probably not terminal, and even if you have diagnosed yourself with whatever type of cancer, or MS or sporadic Huntington's disease etc, you can tell yourself that getting as fit and strong as you can be will massively improve your prognosis.
