She is reactive to sounds, and the jingle of the metal buckle on the cinch with the sensation of it flopping down on her side is what caused that explosive reaction with the saddle. I thought I had prepared her for that but apparently not.
IF she can SEE another horse that isn't afraid of the sound and feed that horse carrots while you jingle the metal cinch she will learn that it won't hurt her. Unlike training, where you try to repeat only 5-7x, you can feed the other horse a whole package of carrots while jingling the buckle of the cinch, AS LONG AS she see and hears you doing it. She WILL be interested and will overcome her fears and YOU won't be right next to her while she spooks. Just make sure that she is in another stall (within earshot/eyeshot) or on the other side of a fence.
When we started CW Reenacting in 1985 we bought a 15yo TW/QH cross. Mother was tired of caring for daughter's backyard horse but he was very safe and not afraid of anything. ALL of his local previous owners came out of the woodwork to talk to me (and take another ride on him.) Among other things, he had been a pony at a racetrack.
He was the toughest horse I ever met or owned AND the herd leader. ALL of the my other horses gained their confidence from him and I was able to break in 15 horses to gunfire BECAUSE of him.
Also, Ryan Gingerich is a gentle, but firm natural horseman. In his program at clinics he tells you to watch HIM, not the horse, bc the horse is listening to him, learning, and reacting positively.
We have to make sure that we are a HORSE herd leader and not a WOLF herd leader. Watch a cat or dog stalk something. They move slowly, and creep. Horses don't like this. The leader of the herd moves towards another member and that member moves away. When the other horse doesn't react, the herd leader pins his ears or raises her back leg. RARELY is there a fight, but when there is both horses will give it their all, so we don't want to get into a battle of the brawn. IMHO, a horse can be rehabilitated into a good citizen, but a dog cannot, so she is worth the effort!
I think that you are making great progress. I broke my arm being thrown by a horse that I didn't ready to the event. He spooked, he bucked, I bailed, and my whole 2004 riding season was a bust. Good luck!
I think this might help you:
https://www.amazon.com/Beyond-Whisper-Training-Language-Behaviorist/dp/B005DI9UBO