I just got done riding her today, and she did really well. We worked a lot on halt-to-walk transitions using my legs, and she seems to understand that :squeeze: is the cue for walk. We made a little progress on the walk-to-trot transition, but she still needs a lot of work on it.
I started off by halting her, then squeezing. If she didn't respond to that, I'd kick her, and if she didn't respond to that, I'd use the crop and voice command at the same time. By the end of the hour or so long ride, she understood the walk cue.
She was also a lot better about staying in the trot. She still dropped out of it a lot, but she stayed in that gait a lot longer than she ever has before, and there were a few times where she kept going until I actually told her to walk.
We both still need a lot of work, but we made a huge amount of progress in just this one session. Thanks for all the advice, and I'll keep you guys updated.