I posted in your other thread about vertical flexion. If you can get her to stop while maintaining vertical flexion and then backing up a sliding stop will start to form, but don't just rush into it. First, start by backing her up when you stop. Then work on getting her to stop by just feeling your body so you can stop her without reins even at the lope. Then try to get her to stop while maintaining vertical flexion at the walk, and then the trot, then lope. Then repeat that last step, but add a back up after you stop, and she should back with vertical flexion as well.
Hopefully that helps
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