@Jenny TipsforME
You are describing multiple problems linked to autonomic dysfunction, and you are dealing with a medical professional who concentrates on heart problems. This is not
primarily a heart problem, but there is plenty of evidence of disturbed hemodynamics under conditions that don't show up in the clinic. (How many patients take hot baths with a doctor watching?)
Reduced blood flow exacerbates many problems with fluids, electrolytes, oxygen and glucose. I suspect that some of your symptoms would go away if your primary problem was corrected, but at this point I don't know how to do that.
I've heard doctors tell me what they think they know about POTS, and it is discouraging, because they are usually badly misinformed. Many concentrate on one aspect of POTS, tachycardia, and treat that as if it were in isolation. (There are patients like myself who do not experience tachycardia, but do show neurally-mediated hypotension over a period of minutes while standing. Even that term is misunderstood because the problem is not usually low systolic blood pressure, but low pulse pressure, systolic - diastolic.)
The primary concern of cardiologists dealing with autonomic problems is distinguishing events like vasovagal syncope from a true heart attack. Once they know that it will not lead to a death which can be blamed on them they lose the sharp focus of attention. Some will fall back on traditional explanations of psychosomatic illness for things they don't understand and can't control. (Many cardiologists could be described as control freaks.)
Even those who believe the patient has a real physiological disease do not have much training in dealing with chronic disease. The strong emphasis in that specialization is on avoiding or delaying mortality, and many patients with true heart disease end up in a morbid condition where they aren't good for much except lying in bed, possibly waiting for a heart transplant. You are in better shape than that, so your current condition is considered acceptable.
Asking a doctor with advanced training in such a specialization, and years of experience with patients who did end up as I've described, to reassess his/her career contribution to patient welfare is too much. If you considered those outcomes as failures the mental burden would be crushing.