It sounds a lot like you are suffering from anxiety
and panic attacks.
Panic attacks can be very scary and almost always involve rapid beating of the heart.
If your doctor found nothing wrong with your heart then trust the doc. and stop worrying about it failing.
Yes heart breaks after break ups can felt physically in the heart but are never dangerous and will never really literally break the hearts of young people.
But I do believe that in couples that have been together for decades, the death of one partner can lead to the other partner not wanting to live and then dieing of a "broken " heart