@aaronblack Hi Aaron and I add my welcome also. I definitely know how what I used to call Phantom feelings are and seem so real
I have suffered with anxiety myself. Min was also due to my Heart but as I became more aware of my body I would overthink things. What helped me is a couple of things first I can to the realization that right where your heart is there is also your stomach, lungs, and muscle. So instead of always thinking it was my heart I would look at a few things. One I got good at checking my pulse and it it seemed normal then I would chock it up to one of the other things. Usually that set my mind at ease. But also you mentioned lying down. Well I do have reflux and lying down makes it easier for that feeling to be noticed and trust me it can feel like it your heart. I hope that helps and if it continues maybe mention to your Regular doctor to check some of the other areas.
Have a Blessed Day
Dana