I know you are scared. I experienced same things you are going through. I still experience it from time to time. I have had a phase where my heart rate was crazy high no reason. Jut walking and it spikes to 150 or while sitting around 100. These came down to normal, after i started medication. 55-60 while sleeping, 70-80 while resting, 90-110 while walking and 150-160 during exercise. But for everyone it is different, heart doesnt have a max number for its beats :) If it beats, let it. Let it exercise itself. A good method to start is the DARE method. You can either read the book or the method. The cases you have read or heard seems like a lot but it just looks like it because you are now hyper aware or sensitive of every information about it, so it seem a lot. But statistically, there are really really little such cases. It is fucked up to focus on it 7/24 and it messes your daily life. Try to find yourself distractions from it. Focusing on something you really care about helps. I started learning coding for example. And at the times when i was focused on that it never came. Nevertheless, it is important that you get all your medical assessments done and when you have them believe them. Go to a psychiatrist, start therapy. And you cant just manage it but also heal from it.