I would just like to add that keeping your hands low is NOT a reward for the horse as somebody mentioned above. This is a common misconception, and often I find myself making the mistake of dropping my hands too low to try to have "soft" hands. In fact, when your hands are too low, this is actually harsher on the horse's mouth as it pulls the bit down into their mouth. Something to keep in mind!
One thing I see that has been mentioned is that your leg COULD come back a tad. It's almost in that perfect line but just a TINY bit off, which may cause instability in the posting trot?? Since it's off by so little, this may not be the case. Overall, you look like a very responsive, and lovely rider. I think you ride very nicely with your horse-what a nice match for you! Your shoulders and overall body position are almost text-book perfect, I'd say. Good work!
Just a side note: be careful about shortening too much, often in shortening the stirrups it will bring the leg forward into a chairseat, not back underneath you.
A horse is the projection of peoples' views about themselves--strong, powerful, beautiful--and it has the capability of giving us escape from our mundane existance.