Okay first of all you are not a hypochondriac I would seriously find a different doctor. I had been told my whole life that I had panic disorder or was a hypochondriac. and last month I got two surgeries in a row and a serious sepsis infection and now all of a sudden I have pots syndrome... it definitely sounds to me like you have something wrong with your central nervous system autonomic nervous system I would try to find a neurologist or even a geneticist.. don't give up sometimes it takes a while to get a correct diagnosis
Your first paragraph grabbed me, because it could have been written by me! The dizziness and breathlessness and the part about stopping breathing while falling asleep! I deal with it too and have no answers, been to heart dr, family dr., neuro dr, ENT dr! Nothing, no one can give me a reason why! I also have difficulty swallowing at times, almost like I forget how, or it just don't work when it gets time to swallow. Not all the time, like the others it comes and goes. I'm almost convinced it's anxiety now because of no answers after years! I hope you find a answer, it might help others like me to know what to look for!
Go to a new doctor, seriously if at any point they laugh off your symptoms they can't treat you any more. The first doc I went to said anxiety and so did many others. "Your heart rate is up when you stand because your anxious" I heard it all. You no your own body better than anyone.
I would recommend going to a cardiologist and getting a 24hr holster monitor, to measure your heart rate over that time. Also getting a tilt table test and treadmill ecg (can't remember the actual name of that test, but it involved a treadmill lol)
took me at least 18months of not feeling well at all to finally get diagnosed with pots. Mine started with a really bad tachycardia upon standing ( 80bpm sitting to 130ish bpm standing) dizziness, brain fog, nausea and generally feeling unwell. I won't say I gets better quickly as I today still feel crap but I helps to no what you have. It's a long road, best of luck.