I've got POTS and neurally-mediated syncope, as well as OCD, possible Ehlers-Danlos, and a sleep disorder similar to narcolepsy without cataplexy/hypersomnia. It's kind of freakish you mentioned numbness and tingling and memory problems, because I've got those too! The memory problems especially have been awful lately. I've also had doctors try to attribute my symptoms to anxiety, depression (I'm not depressed, which is actually surprising given my situation!), and deconditioning/lack of fitness, which is, of course, a lazy attempt at diagnosis and utterly wrong.
Like Clb75 said, anxiety disorders aren't positional. You can tell doctors you're not overly anxious till you're blue in the face and they can overrule you, but they can't easily deny proof that your heart rate skyrockets while standing and falls as soon as you lie down. Those conversations with doctors always remind of that scene in Star Trek TNG when Data goes to see holographic Freud because he's been having nightmares:
Freud: I believe you are experiencing a classic dismemberment dream. Or in your case, being a mechanical man, a dismantlement dream... Now the image of Counsellor Troi, a female, is devoured by you, clearly indicating an unconscious desire to possess your own mother.
Data: But I do not have a mother.
Freud: Do not interrupt! The knife in its violent connotation suggests a certain feeling of sexual inadequacy.
Data: But I have no sexual desire.
Freud: Ach! Impotence on top of everything! It is all becoming clear to me now. There might be a paper in this.
Data: I do not believe I am being helped by this session.
I had a 48-hour Holter monitor test which showed my heart rate averaged 95 and was over 100 50% of the time. I had a tilt table test and fainted after about 5 minutes. I've been on propranolol for about 4 years now, and it helps prevent the gasping feeling when your heart rate goes up to like 150 from climbing a flight of stairs, but I've also had to go down to 25 mg/day because I felt like it was making me even more lightheaded. I would say it's only been a minor help, and has certainly not impacted my quality of life much.
I'm glad I'm fairly skinny because I know if I was overweight the doctors would tell me that was the cause of all my problems. Even so, they keep suggesting exercise - like rigorous, running on a treadmill exercise, which I did for a few months, but it didn't help, and they never take into consideration that I do a ton of walking as a mode of transportation. Most people I know are far less active, and they don't have POTS.
It's becoming more and more incapacitating for me (the POTS/neurally-mediated syncope). I can't go shopping anymore unless I know exactly what I want and where to find it, get in, buy it, and get out. Stores are always overheated, and if I have to stand still for a few minutes, I faint. If I keep on the move constantly, and it's nicely air-conditioned (and there's no uphill walking), I do much better. Whatever I do now, I have to carefully consider beforehand whether I'm likely to encounter a situation where I'll be left standing, like in a line-up or crowd that's not moving anywhere, or if I'm going to enter a non-air conditioned environment. If I do get stuck standing in a line, I end up just crouching on the ground despite the looks people give me. I don't care anymore.
One doctor advised me that I have to be my own advocate and pester doctors until they give me the testing or treatment I need, but I'm too tired to do that anymore. I'd spend most of the doctor's visit explaining my complicated health history, explaining what POTS was, explaining how it is I know it's POTS and not anxiety or lack of fitness, only to be brushed off or told there's nothing they can do other than to advise me to drink plenty of water and eat a lot of salt (which I already do). My day is only 10-12 hours long because of the sleep disorder requiring 12-14 hours sleep a night - I don't have time to pester doctors.
I hope you can at least get a diagnosis though. Hopefully your POTS won't get as bad as mine.