How to overcome health anxiety?

I always think I’m gravely sick and dying when I have the most common and simple symptoms. I had a slight pain in my arm (just a quick throb in my wrist area) and I automatically thought I probably had cancer or some kind of heart problem. I’m not kidding, I am quite literally always worried about my health, and besides being a bit overweight for my height, I’m supposedly perfectly healthy. But that doesn’t stop me from worrying about it and it stops me from trying new things for fear of straining myself or falling ill, and I can barely sleep at night. I’m in high school and I live with my parents, and I have told my mom but she has told me multiple times to simply "get over it". Any advice?