An unfortunate hallmark of anxiety is that often, the suffer doesn't believe or can't accept the diagnosis. How many of us are or have been where you are right now, with the mindset of "I just can't believe?" All of us, at one point or another—FOR SURE.
Hopefully, as time goes on, and you either get enough tests or your condition doesn't get worse, you'll start to feel like, hey maybe I don't have POTS or a hormonal imbalance, and the idea of anxiety will sort of become more valid in your mind. I don't know if that's how it works for everyone, but that's what happened to me. I, like you, was CONVINCED I had a myriad of diseases, but after a while, none of my predicted, assured diseases came to fruition, and eventually, I could get my mind around the fact that maybe what I was going through WAS anxiety.
It's a process. It's all a process. May your's be swift.