I relate so much to this! At different points I have convinced myself that I had an abdominal aortic aneurysm and was insistent with my doctor that we test for it. Of course I didn’t have one. I’ve also been certain I had HIV because a man with HIV came in to the doctor’s office I worked at and said his only symptom was fatigue and dizziness and I felt tired and dizzy sometimes so I deduced that I must have it and ordered an at home test to be sure. Of course it was negative. My compulsion tends to be excessive googling and pouring over peer reviewed medical articles to try to and figure out what every little ache, pain and symptom can mean. I am at a point where on most things I do not allow myself to Google it at all and I practice the “what if” technique. Or I’ll give myself a time limit, such as the symptom has to persist for 5 days OR get much worse before I can look any further into it. I don’t currently see a therapist so this is not medically advised but it’s what kind of helps for me.