Your video does give away that you look terribly tense. Almost as if you're teetering on a scale. You are looking down which will influence that, but are you nervous while riding at all?
One of my favorite exercises to fix the leaning forward problem (which I used to have badly), is at any gait, lean all the way forward. As far as is possible for you and stay there for a trip or two around the ring. Then do the same thing, but lean all the way back. An instructor in college had me do this and it really changed my perspective on where I was sitting and how it affected my balance.
"Donít be the rider who gallops all night and never sees the horse that is beneath him."