You're stressed out, which means she's stressed out, and the stress just grows between you two!
I hit a rough patch with my gelding where all of the sudden, he wouldn't pick up his left lead. Saw a chiro, had a friend ride him (who could have him pick up the left lead every time), worked on it every day... and that was my problem! I was so worked up and so "in my head" about this problem that he started to dread work. We'd work to the right and as soon as we changed direction to work the left, he'd refuse, stamp, back up, hop... and we'd have a little argument at the same spot every lap. One day I realized that I was stressing him out. He wasn't being "bad," he just hated the anxiety of not getting it right! Some horses take things very personally.
So, instead of whapping him every time he refused, I decided to lean forward, pat him on the neck, smile and thank him for trying. Sure enough he'd move on after that every time! When he picked up the correct lead, we'd leave the arena and go walk around the property. Or, I'd pocked a few carrot chunks and make him do neck stretches around to me to get them after he did something correct (man did he love incorporating food into work!). I took a lesson to receive some outside input on our problem, too. For a few weeks after I adjusted my attitude, Jax would hesitate at the "spot" when we were going to the left, and I would encourage him first, then tell him second if that didn't work. Now we have no lead problems and no anxiety in the arena!
Just consider that (1) this is fun! relax! and (2) sometimes your horse wants to be appreciated for all that he does.