I am not going to pretend I have this figured out.
But after years of trial and error I do have a short list of things that actually help me when my body decides it is done cooperating for the day.
Heat. My heating pad is non-negotiable. Heat helps my joints stop screaming and it helps my muscles stop bracing. If you do not have a good heating pad, that is the first thing I would fix.
Salt and water. POTS is brutal and my body does not hold onto fluids the way it should. On a bad day I am drinking electrolytes and salting everything and it genuinely makes a difference in whether I can stand up without my heart rate going through the roof.
Doing less than I think I should. This one is hard. I am a person who wants to do things. But pushing through on a bad flare day does not make me stronger. It just makes tomorrow worse.
Asking for help. Also hard. Also necessary.
Compression. I have a love-hate relationship with compression gear but on the days my joints are unstable it is the difference between functioning and not.
None of this is a cure. All of it just makes the bad days a little more survivable. And sometimes that is the whole goal.