Need advice on getting POTS diagnosis

seeing a neurologist would probably be your best bet. cardiologists don't tend to do any of the testing for it (in my experience) despite the condition being about tachycardia, because it's not an abnormality with the heart itself - the tachycardia in pots is inappropriate sinus tachycardia (or basically, your body telling your heart to go fast because your autonomous nervous system gets confused). a tilt table test is the gold standard as far as i know, and a neurologist who treats POTS can at least send you to the right place if not do it themselves, or recommend other testing as they see fit. you will probably need a referral to the neurologist from a primary care provider - i did at least. good luck to you!