If this is all your GP did after giving this history, you need to change doctors. You should be able to do some easy exercise without these symptoms. At a minimum, you need some basic blood work.
If the measurable physical stuff is ruled out, I’d seriously consider a non-physical or at least non-measurable problem. Chronic fatigue, long COVID, burnout etc. All those types of diagnoses which don’t show up on a test but which are nevertheless plenty real and usually relate to stress in some way or other. You may need to have a rest and then very gradually resume what would feel like comically easy riding, slowly building back up. For instance 10 minutes. If that doesn’t bring on symptoms, next time do 10% more. And so on.
Don’t worry about the racing. Get healthy first and then see where you are.