The amount of times I have cried in my doctors room, or The surgery reception as the kindly hand a box of tissues to me... I think it is because we are generally in a heightened state of anxiety anyway,and for yourself to come away without much of an explanation or a further referral, it's just bloody upsetting. I am so sorry your hopes have been dashed yet again.
Many years ago I was diagnosed with a benign heart murmer. I also have a low pulse rate. I often have my heart skip a beat which is very unnerving. It makes me cough, I don't know why, then rights itself. When I am very tired, or have bouts of anaemia, I seem a lot more aware of my heartbeat, and also short of breath. (Heart disease is rife in my paternal family. That's my anxiety relating to this)
Rightly or wrongly, I accept this as normal for my body. I try not to Google ( mostly) and think back to when I was told the heart murmur was benign.Even if that was 30 years ago. My anxiety seems to be less physical health related and more the mental side and just coping without in general. Maybe this is why I just try to get On with it?
But tbh, I have lived with this for so long it seems part of me, whereas for you it is so recent. And I fully get the need to have a diagnosis of just a definite NO, all is ok.
Hope someone can help better than I can, but I do feel for you.