Make sure when you want her to slow down or stop you step in front of her driveline (the girthline). If you were using a whip I would say move it to the hand closest to your mare's nose and jiggle it to create pressure ahead of the driveline and influence her to stop. If you are circling counterclockwise, lungeline in your left hand, switch the line to your right hand and take a giant step out of the "triangle" to the left, holding your left hand out and firmly saying your "downshift" walk cue. If your mare does not respond the first time, continue holding your hand out and jiggle the lungeline, putting more energy ahead of the driveline.
I usually start this process using some kind of whip or stick to give my arm more reach, and the horse gets used to it enough to not need the whip, only the movement of my hand or the verbal cue.
Hope that helps. Good luck!
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown