Hey,
yeah I used to be worried that my heart would stop, usually at night time. Actually it used to be at pretty much the same time every night for awhile (around 3am) and I would wake up pretty freaked out!
I try not too worry about it too much anymore. I just tell myself that theres no use worrying about it and I'm better off thinking about other things. Sometimes I try and think about the fact that at the very moment I'm worrying about my heart stopping it is actually still going, and going pretty strong! I try and remind myself that when our hearts speed up during panic attacks, its no worse than doing a bit of exercise.
hope you are all going well
take care,
Kieran