One day, I went to check my pulse. I accidentally touched my collar bone, not my vein. I instantly panicked and thought that my heart stopped. Of course, I moved my finger nearly instantly and felt my pulse. Didnt matter. Two years later it still gets me now and then. over literally nothing.
I can tell you exactly whats happening, but I don't know if it'll help.
You get scared, and you check your pulse rate. You see its normal. This makes you calm down, which makes your heart rate drop. But because you're scared of your heart stopping, this triggers massive anxiety, which floods you wiht adrenaline, which causes your heart to beat irregularly and speed up. And then you inevitably check it again, and the entire process repeats itself.
If you stop checking, you won't be afraid. After years of experience, that's the only thing that stops it from escalating. Do not check your heart rate. You'll just terrify yourself more.