I know this is easier said than done. Mac used to spin on me when out on the trail; very suddenly, with NO discernable warning. He spun me off FIVE time! I got kinda tired of that and started to "expect" it. So, I was not fully committed when we trotted down the trail. To this day, I know that my worry is felt by him, even when I try my darndest to cowgirl up and not show a whiff of fear. So, when Mac's owner rides him, someone who he has never spun off, she rides forward with the expectation that he won't spin her off, . . Voila! He doesnt.
My point being that it's very hard to erase the memories of bad experiences, for both horse AND rider. But, if she were ridden by someone for whom she spooks less, she would gain confidence and I think you'd do so, too and you each bolster each other.