I spent some time riding walkers and know what you're going through. Most of it was fixed by riding without stirrups. When you ride saddle seat you brace with your legs.
Concentrate on having a good open bend in your knee and riding through your core. I took a lot of short vids so I could reveiw them right there while riding. It may help it you shorten your stirrups too. I found that if I rode with them shorter, I was less likely to brace.
There was also an exercise my trainer had me do. It's a runner's stretch but you do it on the horse. Take your foot out of the stirrup, lift your knee till your thigh is parallel with the ground and rotate it out to the side and back, keeping the leg parallel. It's supposed to open up your hip.
If your horse says no, you either asked the wrong question or asked the question wrong
And God took a handful of southerly wind, blew His breath over it and created the horse