Ugh. I feel for you. A jigging, pacing, just get there horse is a very frustrating experience. While I agree with others, that the bit is not the answer. With your mare, I would start with some very simple exercises. It sounds like when she acts up, she's already a few steps ahead of you. She knows that when she does A, you'll counter with B. She's already thinking about C,D and E, and I'm guessing you just end up a tense, frustrated rider who really just wants to get back to the barn and get off, ironically same as her.
I would start in the ring with some very simple exercises. Break it down into one cause/one reaction concepts so she forgets about escalation. The first game I would play is walk/stop on a long rein. Let her walk freely and follow her motion completely with your seat. At your choice, ask her to stop by not following her motion with your seat, tighten your stomach mucles and sit tall and deep. At first you will have to use your reins to let her know what you want, but once she starts understanding what you want, try not to use them. The important thing with the game, and really all of them, is she has to do it correctly even if only once. It might take 10 minutes or an hour, so make sure you have time to follow through. When you succeed in stopping her with your seat, game #2 starts. Game #2 is she doesn't move forward until you say so. At first increase her chance of success by only making her stand a few seconds before moving on, but if she moves off prematurely, seat stop again. If she gets fussy, try as hard as possible not to resort to the reins. Sit deep and tall and make as little a deal about it as possible. Once you get start/stop down, try it near but outside of the ring. If your training is solid, walls shouldn't make a difference. Again, don't stop the lesson until she's done it correctly, even if it's only once. When she does it right, praise her big time. If she gets jiggy, act like it's all good and you have all day. Don't react to what she's doing, but do manage it. Use leg yeild, shoulder-in, haunches-in, whatever you need keep her attention until you can get it back to just a basic start/stop game. Mare's can be a little thick, but ultimately she needs you up there calling the shots. Good luck.