Like everyone else has said, you just need to work on it a bit.
Judging from the video and the photo, you seem to sit up quite high when you jump. I would suggest practising with crosspoles - they can be small as you like, but most horses still tend to do a nice round jump over them - and make you fold rather than just leaning forward. Also, in the photo, your legs have gone back slightly and your heels are up. Maybe this is just a one off, but just remember to keep the heels firmly down - that nearly always helps prevent the leg swinging back, which then stabilises the upper body.
Also, something I only actually learnt recently is that as you land, and for a stride you two after the fence, you should remain in the "folded" position - this allows the horse to get his back end clear over the fence (although I doubt that's going to be a problem for your horse anyway!) and allows him to gather himself after landing. It will also make it a lot easier for you to return to an upright position or light seat as opposed to being "thrown" back as you come down from the fence.
Have you ever read any of Pat Burgess's
training techniques? She explains it all very well, you might find it useful :)