I had the same exact problem and still kina of do but she's better now. Her withers are very sensitive so the saddle wasn't fitting correctly. So i suggest look for a new saddle.
I would try desensitizing her more to it practice throwing the saddle on and of cinching her up and And keep doing that hat until she is ok with it
It's could be because of saddle fit and now she associates anything to do with a saddle with pain. It could also be they have stomach ulcers, I knew a horse who would get irritated during the tacking up process because he started to associate it with pain
Same exact thing with my ottb in January- u really just gotta work at it and now my mare is perfect with it- try having someone hold your horse and one on each side to tack up
Could you try desensitizeing her to the pad or blanket. Or use the clicker training show her that there is a positive relationship between being saddled and ridden
And she is sound. But is there anything we can try like saddling from the offside?