Hey Neuro, Ya know when I first got sick with my autonomic disorder and got my pacemaker I had this same anxiety problem...focused on my heart stopping. Funny thing is that when you concentrate on it you may not feel your heart beating. After I read your post I sat here and just tried to tune into my body for a few minutes and feel my heart beating....I couldnt. But keep in mind that I have bradycardia and have a pacemaker to keep it regulated so, I know now it isnt going to slow down in the 20's like it did before.
Now, it is always possible for your heart to just stop beating but I imagine that there would have to be some serious heart diease or defect going on. What may be going on is that your heart is beating at a slower rhytem which would make it difficult for you to detect your own heart beat.
Have you seen anyone about anxiety issues? It took me a long time after all my heart problems were diagnosed to come to terms with it and finally get to the point where I didnt obsess about it, or my pacemaker. I am not saying that your obessing or that this is even anxiety...If you have a concern that you have a medical problem going on have it checked out. I just wanted to point out that lots of people when faced with heart problems in comes the anxiety which can be just as bad and doesnt help matters. Okay I am just rambling here... 
I hope u were able to make some sense of my crazy post. Take care dear 