I have Lyme and go quite often. It can get extremely exhausting, so I always have a contingency plan in the back of my mind wherever I am so that if I need to rest or get out of the crowd I know where that place is.
The issues that I have the most problems with are:
Mobility: I use a rolling walker with a seat so that I can lean and sit, this helps a lot. I would use a
scooter, but the anxiety of trying to drive one in the crowd would be too much for me.
Anxiety: Being surrounded by people makes me very uncomfortable and can set off panic attacks (never had these before Lyme). My best advice for this is good planning, arrive early if possible, use fast passes and the return time cards, eat meals when the restaurants first open (this will allow for no lines and ample tables), park hop if the park is more crowded than expected (another one may be less crowded).
Sound: Certain sounds can really make my anxiety/stress/pain skyrocket. I don't always know what/when they will be, but at Disney there are lots of places that are loud (general crowd noise, music, rides) so I always carry a pair of ear plugs with me. It looks kinda silly but it really helps me so I don't care how it looks. Sometimes I will use headphones and listen to something that I like (music, audiobooks, radio) but at Disney that shuts out the magic. With ear plugs you can still hear what going on around you, just at a lower level.
Heat/Sweat: Sometimes I will carry a wash cloth with me (just in a zip lock back in my pocket/bag) and soak it in cool water to drape around my neck or wipe my face to cool down. I also take a small dry sack with a couple of zip lock bags lining it so that I can fill it with ice to have an ice pack anytime I need it. Sometimes restaurants will fill it for you, but if not first aid will. First aid will also make one for you if you do not have your own bag.
Hope this helps, have fun!