I am on my phone, so will add more later.
Why is she in a snaffle? At least it looks like it from what I can see in the video. How old is she?
Everything in your position is pitching her forward and making her get quick. Sit down and back. Think about making the front part of your pelvic bones the farthest forward point of your body. Bring your upper body back and roll your shoulders back. Bring your leg back, open and relax your thigh, and quit breaking your ankles with your foot out. Put your weight in your heel, turn your toes to the outside, and put your entire leg and your foot against her.
Get off the rail for now. I'll give you some more info later.
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