I have seen several of the videos of you on Bee, ones that you have been so kind to share and so brave to open to critique. I think you will be a dandy rider, soon, but there is some real stability missing. I think that it is that you are not really letting hyour weight sink well down into your legs, letting them drape down and around your horse, and you are stiff in the hips, so your upper body has trouble remaining free from the motion of the horse becuase your hip is not moving enought to absorb it before it moves up into your upper body.
I would really recomment that you spend some time riding either bareback or stirrupless with your leg draping down. to be able to ride this way you absolutely must be able to balance on your seatbones. Grip alone will not save you from falling off. Grip of the knee or worse, the calf, is what ends up flinging you forward when the horse stops or changes direction suddenly.
Another possiblity would be to ride western for awhile and work on riding with that long, draping leg and sitting right over those seatbones. Once you get this down you will be a really cute rider!