Have you have been seeing your doctor regularly, had your cholesterol levels tested, and had a colonoscopy recently? These will address any possible health issues you may have, and/or reassure you that you're OK. Medicine has advanced significantly during your lifetime vs. your parents, so there is every reason to believe you will enjoy a longer life. But of course you have to have regular visits with the doctor for that to happen. Consider getting a referral to a cardiologist who specializes in lipidology. He/she will carefully determine your HDL, LDL, triglycerides, etc and suggest a course of action (like a statin) that will help prevent a cardiac event like your mom had. And a colonoscopy will help prevent colon cancer like your father had. Taking an active role in maintaining your health should reduce your panic attacks because you'll have a concrete idea of where you are at healthwise, rather than always imagining the worst. I hope that helps!
Im 26 and have all these worries and have had these since i was about 10... As do most others with anxiety. But i keep telling myself we all die eventually. What can i do to prevent that... nothing.. so what is the point i worry about something out of my control. I could go out today and be hit by a car. I could die at the ripe old age of 90. No one knows.. we just have to try and live life and enjoy what time we have. Which is easier said than done.