I also have the problem of having my mind latch onto something obsessive like my heart beat, it will sometimes keep me from falling asleep/keep me awake, bother me to the point of just lying bed waiting for *doom*. I have tried hard to stop thinking obsessive thoughts, every time I started to think it I would stop myself. It's hard, but maybe you could try that, just tell yourself three to ten times that you are okay, your heart is fine every time you have one thought that it isn't fine. It is super hard, but I feel I talked myself into thinking things repeatedly so I best pick the good things to think.
I'm in therapy and is helping, although a slow process, so now I have days when I don't have problems with obsessing over my heart, although some days like today I still do. Just be careful with trying to rid yourself of the heart obsession, I'm finding that as I'm trying to stop worrying about that other worries are taking over. Like going blind, going deaf, my knee ached a bit after a workout and suddenly my mind leapt on the 'for sure omg' my knee will have to be replaced and I'll be crippled for life. It is very hard to explain to people who don't understand, which is one reason my therapist is really great, and now these forums are too.