Kawaii, it's hard to tell from still photos, but it looks like your seat is pretty stuck to the saddle, so it can't be that bad. Can you ride sitting trot comfortably without feeling like your being bounced all over the place? When I learned to sit for dressage after years of riding hunter-style, my trainer told me to think of tucking my butt in underneath me, and relaxing my back. Your hands are very low, but there is also a nice straight line from bit to elbow. However, when his head is flexed he tends to overflex. So Sitting deeper and driving more from seat and leg (as well as bringing your hands higher) should help him elevate into a truer flexion. About your leg moving forward when you lengthen your stirrup - that's pretty normal in a jumping saddle >.<. When you go no stirrups, focus on keeping your hips tilted back, your butt tucked underneath, and work on the flexibility in your hips by stretching your leg down as long as possible.