This is a difficult topic for me to articulate, so the below explanation is merely my best effort to express the experience in words.
The perception of my own heartbeat is something that I've always been squeamish about, but for most of my life it was never a problem. It made me a bit uncomfortable during high intensity exercise, but it was easy enough to just ignore.
During meditation, I began to perceive my heartbeat more intensely than ever before, and it became a "center of gravity" that my attention would often "fall into". My attention would gravitate toward intense perception of my heartbeat not only while meditating, but also at other times (while trying to fall asleep, while trying to focus on work, while driving), and paired with my pre-existing squeamishness, this became very uncomfortable. I might go through most of my day normally, but as soon as my mind becomes idle for a bit, or if something triggers me to start thinking about my heart, my mind will uncontrollably gravitate toward perceiving my heartbeat, and I'll struggle to ignore my heartbeat and to focus on other things.
My strategy at the time was to attempt to become more comfortable with my heartbeat through continued meditation. I thought that by deliberately focusing on my heart during meditation rather than trying to ignore it, I could "fight the monster face-to-face", kill off the squeamish feeling, and learn to perceive my heartbeat as a benign sensation.
Unfortunately, I developed a delusion that I might gain the ability to consciously control my heartbeat (analogously to how we can consciously control our breathing when we think about it), and that I'd injure or kill myself because I'm not at all qualified to exercise that kind of control. I convinced myself that this is impossible because the heart uses its own pacemaker, unlike the lungs which are controlled by the nervous system. But this didn't kill the delusion - it just transformed it into a more vague anxiety, centered around the notion that I might be inadvertently abusing whatever regulatory connections exist between my heart and my brain. I think this delusion is really just a transformation of the visceral squeamishness that I originally felt when I began perceiving my heartbeat, into a more cerebral form of "squeamishness". I made the decision to stop meditating a couple years ago, but the delusion still lurks in my subconscious and comes back from time to time.
So to summarize, when it first began, the uncontrollable heartbeat perception paired with the squeamishness/delusion caused a lot of agony for about two weeks. I've gotten significantly better at ignoring my heartbeat and not being so troubled by it, but it's a problem that I haven't been able to completely get over.