Apparently she really needs a medium/narrow to clear those shark fin withers properly, and when she has one she rides like a dream.
She was so much better today than she has ever been. No head tossing, no running into the fence, no bolting. She even cantered some circles for me and that's not something we've been able to do without being in the round pen.
Simply put, she was an angel. Yes, I was one of those people that thought "my saddle is fine, that's not the problem, it's the horse being bratty." But no, it really was the saddle. I feel badly now thinking that my horse was probably in pain or uncomfortable this entire time. Live and learn. I will always pay proper attention to saddle fit from now on.
If you think your saddle fits but are having other issues, double check it!