good grief she took off long! You leg is very still and solid on the flat. I couldn't see much of your hands due to the video quality, and the big jacket may be hiding a perfectly straight back (it makes you look hunched, but I think it is because it is poofy).
I've jumped ponies and I find it hard to keep my leg on properly after riding a big horse, so I think you are doing a good job considering the situation she put you in. If this resembles how you always jump I'd recomend staying up out of the saddle long and not sitting on the horse over the fence. However, given your solid lower leg I'd say it was a matter of taking off long and trying to save yourself lol.