I had the same problem with my riser pad hurting the back of my boy. I had a saddle fitter come and look and she said I need a saddle with real wool flocking ( mine had foam) that way for $65 any saddle fitter can fill the saddle in with wool where it needs to be so it sits just right on the horses back. The nice thing about a real wool flocked saddle too is that once your horses back changes the flocking can be moved or soem taken out to readjust for the new muscles.
JMO. Hope things get better for your mare soon.
I am owned by the Sandman