This may not be relevant to your situation, but my WB had a very on/off progress in his early years of training. He was smart and willing and I had to keep introducing new things because I didn’t want him deciding that where we were was how things were done. Especially in dressage we are asking for things to get better in increments.
Sometimes that meant three steps of a thing. Keeping the hard things short, and praising the improvement I looked for without asking it to continue. My horse wanted to do the right thing and was anxious about new things, asking for reassurances when I told him he got it right. As his confidence and strength for the thing grew he would puff up proudly and continue doing it when I told him he got it right.
This looked like teach the thing (LY, SI, etc) and then use the thing to address specific weakness. That might mean going from trot to walk to use a few steps of SF to fix a dropped shoulder, returning to trot, and asking for SF in trot when the shoulder dropped again, and dropping back to walk if needed to get the SF. Not in a rapid fire way, only as quickly as he could mentally handle. It looked like patterns that had frequent changes of direction built in, which prevented overworking muscles and would help interrupt resistance through the body (101 Dressage Exercises - Needlepoint is a great example as gait and position can be varied almost endlessly within the pattern).
When I finally managed to get the stuff that was constantly interrupting his progress sorted out, I was surprised at just how much he did know and how easily he could put it together.
I’m sure you know the theory of doing the hard thing for a short time between lots of breaks doing easy things, but maybe your short time isn’t short enough for her. Your mare might also be getting stuck in the mindset of thinking she knows how things are supposed to go better than you do.
My lease horse was like that after a year of WTC with a barn kid who didn’t know more themselves. This horse kept telling me I was wrong and that what I was asking (trot and bend simultaneously) was impossible. Once he figured out that I was wanting the different thing, and he could do it, he opened up and tried to figure out what I was asking for when I asked for something new.
I have met horses who know their job and were not open to changing any part of it. I think that’s something we have to be careful to avoid with these smart youngsters when riding the line between building the strength and asking for more. Making the easier things harder by doing something like riding patterns on sloped ground can help by giving built in opportunities to do things like balance and rate speed going downslope, and balance and push going upslope.
