I am so sorry you're going through this.
I don't have POTS, but I do occasionally struggle with orthostatic hypotension (no tachycardia) for various reasons, so I can relate a tiny bit. My tendency to get the vapors like a dainty little 1800s lady didn't affect my medical education at all during M1-M2. However, it has had a minor impact now that I'm in my clerkship years. I have had to ask for a few accommodations when I have weeks where it's bad (e.g., letting an attending know I may need to sit down on rounds if I feel an episode coming on), but everyone's been great about it.
If your symptoms aren't very severe, you could be mostly fine in preclinicals (especially if lectures are attendance optional and you can study from your couch). Anatomy might be tricky. Get on the radar of student disability services upfront to arrange for whatever accommodations you'll need in a standing lab environment.
Clerkships could pose an issue if your symptoms aren't well controlled by then, but cross that bridge when you get to it. You can't really predict the future with POTS. Patients often go into spontaneous remission, and you could very well be in a better place two years in the future.
I think there's one thing you need to do that's essential to your success in medical school: make sure you have a good counselor or therapist who has experience treating patients with chronic illness. I cannot emphasize how important this will be for you. Medical students with chronic illness can absolutely thrive and achieve all their goals in medical school, but virtually all of them have strong support systems in place. Start thinking about what that's going to look like, if you haven't already done so. That's my advice to you.
Good luck. I think you can do this.
