Reward her when she stands still even if it's for a short amount of time and gradually lengthen the time she has to stand still for to earn a treat
You could try keeping a lead shank on the horse while they are in the cross ties or while tied up and have it with you the whole time so if the move you can put them back in place a little bit easier
You could also try having someone else that is experienced and give them the lead shank
I would recremend doing lots of groundwork with that horse to establish that your the boss so she respects you more. Work on moving all her feet individually, so she doesn’t think she can walk all over you like that.
Sometimes you just gotta follow them and continue doing it brushing, grooming, tacking up etc. they will realize that them moving doesn't make you stop doing it, so they will eventually stop. Or it may be the flies. I know my horse always stomps his feet etc if there is a lot of flies so that may also contribute why they move around a lot. Reward them for standing still so they have something to look forward to or know it's pleasant to do the right thing instead of the wrong thing. Make it easier for them to do the right thing and harder for them to do the wrong thing. Horses don't know wrong and right. They just do whatever they want to or whatever is easier for them.