Is she just spooky, or is she very forward when you ride as well?
Every horse is different, of course, but what I have had success with in the past is encouraging “whoa”. A lot of walking, and then picking up a trot. Only using seat, leg, and voice (and circles), keep asking for slower and slower, until the horse eventually breaks into a walk, and praise! Rinse, lather, repeat, and don’t overdo! If you get a couple quiet short trot sessions, I would end there. Eventually, you get five or six trot steps before the horse automatically breaks into the walk, which is when you start adding more leg to continue extending the slow trot session. It may seem like you’re teaching another bad habit, but it’s easier to add a motor than to take one away, and since you’re only softly using your aids, you should not be creating a sour horse.
Same with jumping. Set up a low, low fence, trot up to it, and when you are still far enough away to not be teaching run-outs/refusals, either halt, circle, or bring the horse down to a walk and walk over the fence, then eventually you can do the same canter —> trot. Never thought I’d have the biggest smile on my face when my rushy OTTB cantered out of the corner to a decent-sized jump and broke into a balanced trot, then rocked himself back while I had a soft hand! Only took a couple more schooling fences with a little extra forward encouragement for him to hold a steady, balanced, and consistent canter up to, over, and away himself.
Again every horse is different and I’ve had this work on a couple, but not sure how if it would suit your mare. I figure this teaches the horses just because we’re moving doesn’t mean we’re going anywhere, so what’s the rush? (ETA: kind of implied, but everything is done quietly! Keep your breathing and body slow, even when she’s not wanting to slow down herself. Don’t fight with her for any downward transitions - it just makes her resist. As hard as it may be, give her the idea of the downward transition, and praise her when she “comes up with it herself”
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