I can definitely relate to what you're feeling! I used to be CONVINCED I had some heart defect (sometime it was an arrhythmia, sometimes it was a leaky valve, it often changed) and this persisted for a while and then slowly disappeared as my anxiety and panic attacks decreased. BUT the minute I was panicked or anxious again, that fear of a heart defect would come right back. It's really common for people to have one or a couple persistent fears, when our body is in an anxious state it scans for potential threats as a survival mechanism and when it doesn't see any concrete threats it hones in on bodily sensations...for me a rapid heart rate was a common anxiety symptom so my fears automatically interpreted that as there was something wrong with my heart (even though a fast heart rate is a normal by-product of the body's stress response). Rationally, I was 99% sure it was just anxiety but it was the "what if it's something else" thinking that kept the fear alive for me, all it takes is 1% doubt that it's something other than anxiety to keep that fear alive, no matter how rational you are! Some of the most intelligent and rational people struggle with panic attacks and anxiety, Charles Darwin had severe agoraphobia and panic attacks!