Ok have more time now. . .
I can relate to many of your symptoms. I'm so sorry you're having them. There is hope.
Regarding POTS vs MCAS vs who knows what, it depends on which physician you talk to and what their area is. Some say POTS is secondary to MCAS, some say the other way around, some say entirely separate. The main thing is your symptoms are real and the main goal should be improving them keeping all the possibilities in mind.
It sounds as though you've had an extensive medical work up but perhaps have not yet been seen by a mast cell doctor. The best are in Boston and South Carolina. Not sure where you are but you could also have a local doc relay with them to sort out any extra testing you need. Has mast cell related bloodwork been drawn?
Fatigue was one of my earliest symptoms. What happened with the ablation? Did you react to the contrast? My downard spiral began with CT contrast anaphylaxis. I get facial flushing and burning too and you saying that is sure suspicious of mast cell issues though other things like carcinoid and pheochromocytoma need to be excluded. If you can start keeping a log of everything you eat, if you're overheated, exercise, stress level, meds etc and try to pin point any triggers for your issues that might help. I'm still sorting out all of mine, but the big ones are stress, heat, contrast, hazelnuts (IgE), many raw fruits/veg, scents, cleaners, strong cooking smells etc.
I was bed bound when my POTS was at it's worst as standing would induce hypotension and tachy ~150 bpm. I started eating small meals, drinking heaps of water, got out of the hospital, sleeping loads, and doing an elimination diet and slowwly things improved. I tried to walk or stand as much as I could (staying cool) and this built up gradually and now I'm able to hike and walk for hours which seemed impossible for a time. I tried FLorinef for the POTS but reacted. I tapered off steroids and my POTS improved. My mast cell meds were the same for 6 months before my POTS flare and my POTS symptoms improved gradually after that point with no change in my mast cell meds.
The specific details of what causes what are not entirely clear in the science yet, but it is clear that there are many people with issues like yours of orthostatic tachy and reactions to things like I've mentioned. Whether they're all the same thing or not time will tell, but working at treating each of your symptoms, identifying triggers, and treating the whole picture is key. Seeing a doc who sees you as a whole picture can be so helpful for this.
I would seek a second opinion on the "not allowed to take antihistamines" ideally from a mast cell doc or a knowledgeable POTS expert like Dr. Grub. If this were actually the case I would be very shocked. But do keep us posted as it is not my place to tell you anything is impossible. If they end up being contraindicated and it seems like you have mast cell issues (which would be my gut feeling based on how similar we are!) perhaps mast cell stabilizers and antileukotrienes would be something you could try.
Postprandial hypotension could be part of a mast cell reaction or could aggravate your low intravascular volume by diverting it all to your GI tract worsening your tachycardia even without mast cell issues, or both! What helped mine was the elimination diet, eating small amounts, and drinking lots of water.
Yes there are types of POTS that are unrelated to mast cell disease. Most articles however describe flushing as a red flag to suspect underlying mast cell involvement.
Courage waxes and wanes for me depending on the recency of a scary reaction. The elimintation diet and introducing one new thing at a time really helped with this as you can take it at your own pace.
I ssoo sympathize for your life crumbling before your eyes and being stuck in bed. But keep hope and keep fighting. Try to think of your recovery as a long term process so you dont get too frustrated day to day. Most people find ways to get back to leading semi normal lives.
Last thing, I know it's scary but try not to be terrified of this diagnosis. There is something going on with you that needs treatment whether you name it or not. Whatever the specific names are on paper be it POTS, MCAS, or both it doesn't change that you need a comprehensive treatment plan for meds, trigger avoidance and lifestyle alterations to try and get you back to leading a more enjoyable life. "A rose by any other name would smell as sweet" as they say
Though, I realize nothing about your current situation is rosy or sweet.
You will get there. You are at the most difficult part...the start. But you're doing the right thing to find out everything that you can and seek expert help.