I know how you feel, OP. I have fms too and have struggled at WDW each time. I take ultram 3xday, but can take double that if needed. So, I am on a pain med schedule and I add another one if needed. I also supplement with alleve or ibuprofen at times. If I didn't take these, I couldn't function at all.
Then mornings are best for me, so we do rope drop and go at a slow pace til noon, have a noon meal, then back to the resort for a rest. In the evening, we eat at the resort or when we get to another park or DTD. We go even slower in the pm; no rushing, just go with the flow, see shows, etc.
I have not tried an
ECV yet, too stubborn. And yes, I suffer with a lot of pain each day.
This time I will probably rent an EVC because I now have plantar fasciitis of the foot and walking is difficult.
I guess all I can say is slow down, rest, take pain meds as directed, take many rest stops at the parks. I hope this helps. There is no "golden rule" for those of us with fms so, for me, nothing has really stopped the pain. But the above mentioned has helped with the pain. Good luck and enjoy your trip
