She may have ulcers due to aggravation or stress. I had a horse that did that if not, work ground manners on her for a while until she accepts contact and pressure. She may just be moody, try MARE MAGIC herbal calming supplement
She's been at the farm for a long time, and has been ridden a lot, and I've tried to stop her that way and unfortunately it didn't work. And people tell me that she's been like this forever @nataliaortegaphoto @maliagibert
Have you tried using your body weight? Slow down with the reins while you put your weight back and place your feet a little ahead so he doesn't feel any pressure that can confound him
He probably doesn't have any experience saddle work and if he did it might have been a negative experience, I would work in hand work then do undersaddle let work to let him know it's ok.
I'd say she's just impatient or if she's been in a school for a while, just knows the drill!
She doesn't have issues with transitions such as walk to trot, trot to canter or vice versa. She just wouldn't stand still. Any suggestions to teach her to stop?