You Seem to be leaning backwards, pushing your feet forward which will make your horse hollow out (like in the video)
Square your shoulders up ( try lifting shoulders up, back and down)
Remember ear, hip and heel in line.
Also I think it would be an idea to take up a little more contact.
Having a few lessons with a qualified instructor would be an idea too.
I have very bad balance and I will be working on fixing that(I tend to balance on the reins). <-- At Least You Know That & Want To Fix It Unlike Horse Riders I Know.
Keep Practising & Riding!
Hope That Helps You!
"If you act like you've only got fifteen minutes, it'll take all day. Act like you've got all day and it'll take fifteen minutes." - Monty Roberts