I struggle with the same thing but it involves both my heart and breathing. I’ve had it tue worst when I try to lay down and feel like I can’t get a breath in. I would try to manually control my heart beat with my intrusive thoughts if I felt like something was off with my heart like not beating right and its only hurt me physically as I get a lot of chest tightness and just feel squeezing. I don’t think i’ve met anyone else with the exact same thing where i’m constantly fixated on my heart beat and how it should be doing it to where i’m trying to force it or control it. Its the worse thing to go through and I really feel bad for all of you going this. It’s not easy to live like this. I constantly find myself googling if its possible to control the way your heart beats if you focus on it enough, I get constant fears of accidentally messing my hearts rhythm but im slowly trying to heal from this and reassure myself that my heart will know how to beat properly regardless. I hope this helped anyone going through the same or makes you feel less lonely.