There is a possibility that she may have stifle/ hip/ back issues and may need some chiropractic work. My percheron/ thoroughbred mare used to have a tough time bending and getting into corners with dressage. However, she has weak stifles and needed to develop more muscle in order to push off well with her inside hind leg. I'm looking into getting her messaged and adjusted soon after the holidays as well to really make sure she is fully comfortable before increasing the intensity of our workouts.
Before addressing any training or attitude issues, ALWAYS look into a possible health issue instead. Most comfortable horses will do what you ask them to as long as they understand. It takes a very tolerant horse to do what you ask if they are hurting somewhere.
Originally Posted by PaintingMissy
I also agree with having her stand in the corners. If you do any ground work like lounging you could work her in the corner of the arena which may help.
I sort of agree with this statement, once you have narrowed out any possible health issues
. However, I would make corners a "fun" and relaxing place to be rather than somewhere where she has to work. Lunge her in the middle of the ring and then let her rest in the corners. Pretty soon she will love being in the corners of the ring, since they coordinate with rest and relaxation. This will be an especially good exercise if your mare is on the slower/ lazier side, since the resting will be much more motivational then for a speedy/ flighty horse.
(Random tangent, but this exercise will also work for barn/ pasture sour horses. Work them in their favorite place and then let them relax in the place they don't normally enjoy- like the arena for some horses. Soon the horse will think of the arena as a good place since they get to relax there, and the barn as being not as much fun place. Note, the horse will still naturally like being in the barn, but he won't associate it so much with not working and being a bum!)
Anyway, good luck and let us know how it goes!