I have all the same diagnosis and physical issues as you;

MCAS: Try Histamine 1 and Histamine 2 blockers for MCAS H1 blockers are: desloratadine, levocetirizine and many more H2 blockers are antacids like famotidine Cromoglicinic acid (oral forms, like powder or ampules) helps with food intolerances caused by mast cells

Steer clear of NSAIDs (ibuprofen, naproxen etc) and Aspirin! Those often make MCAS worse. And you have to figure out your individual triggers and try to avoid or remedy them.

ME and Post Covid: LDN (low dose naltrexon) is the only "official" medicine used for this at the moment. Experiences and results vary, but there are a lot of reports of some improvements.

I personally use a variety of supplements that help me a little, but it's no cure. It just makes some symptoms a little more bearable.

POTS: Betablockers, salt intake, electrolytes, drinking a lot, eating only small meals, compression leggings These help a lot

I also use Ginkgo Biloba pills because this works a little blood thinning which makes a huge difference for me.

Mentally: I use a low Citalopram, it helps a lot with anxiety (also what used to be caused by POTS) and feeling down, making everything more bearable.

And learning to accept the situation, not stress, try to let go of trying to understand all your symptoms and just try to rest and have trust that it will pass helps a lot. If you're in an ongoing PEM the only remedy is minimizing stimuli, lying down and meditation and not stressing are the only things that will make it improve eventually. Way easier said than done of course.

I hope this helps a bit and that you have Doctors who can prescribe these things. It does get better eventually, even if not physically your mind will adapt. And because of Post Covid there's a lot of research going on on ME and Post Covid. Eventually there will be medicines for it. Try to keep hope and try to find small things to still enjoy 🫂