Several things hit me.
First you look as if you are riding way to short, secondly you are relying on your stirrups and thirdly I do not think you have ever been taught the importance of your seat.
I agree with lots of no stirrup work. You need to learn to brace your back, am excellent book on this is Riding Logic, Wilhelm Museler, a brilliant book with lots of exercise to practise at home.
When you get on the horse try putting you hand under your thigh from behind and pulling the flesh to the back, this puts your thigh flat against the saddle and knee forward so it is easier to keep your lower leg flat against he horse's barrel.