Aww sweet thing - yes, small circles help slow down/relax nearly every horse… except one that has been trained to explode out of them.
What about the clover exercise with 4 poles? Trot the circles and walk over the poles. Eventually extend the trot around the circles, and collect it over the poles. If she gets too strong at the trot, you could add a couple of poles so it makes her think about her feet some more instead of blasting away to the next circle like she’s been taught to. It’s very easy to collect/steady on a circle and then send them forward when they’re going straight again, but I think you need to focus on doing the opposite.
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I also like the “unplanned” exercise for horses in need of some strengthening - even better if it’s in a ring with a lot of jump standards/poles laying about. Basically just start trotting around and loop around jumps and standards randomly, change directions, weave around, pop over a pole, ride between two poles - whatever pops into your head in the moment. As long as you’re fair and relaxed about it (no cranking around last second), not being able to anticipate what’s next really helps horses learn to balance themselves so they’re ready for whatever direction you end up going.
You might also work turns on the forehand. Rollbacks and turns on the haunches are usually drilled into barrel racers, but turns on the forehand will probably be something new.
I’ve run out of things to do at work today, so now I’m just spitballing about what I’d do to retrain a barrel horse. Feel free to ignore me completely 