In your situation:
1. I am asking you to please have a qualified chiropractor look at her.
2. If the chiro declares her sound, check your saddle.
3. If the saddle fits correctly, I am sorry but check yourself - lol lol
4. I have had Spondoliothesis for years and years and years -- I am now at Grade III, which interprets to 50% - 75% disabled and I rarely ride these days.
For years, my friends would tell me to sit up straight that I was leaning -- I would get huffy and tell them I WAS sitting straight, my eyes were looking straight between both ears.
Thennnn I looked at some pictures that were taken while riding all my horses. I could've cried because I was sitting crooked, even though my curved and deteriorated spine told me, in error, that I was sitting straight.
All these years of riding (I've been slowly deteriorating since a bad accident when I was 12), and my horses have somehow managed to compensate for my serious shortcoming. It's a good thing I never aspired to go into the show ring
Have someone get a picture, of you on the mare while in motion. They need to be standing squarely in front of her, in order to show your position.
5. If all of the above prove fruitless (although my money is on needing a chiro), then she needs some work
Hope this helps