Your legs are a little far back, but not by much. About your back, it's fine... it's your shoulders that you need to fix. Push your shoulders back, as if you were swimming.
When trotting: Your heels should be down a bit more, and pointing more forwards. Right now they are pointing outwards, which causes your knee to come off the saddle and the wrong part of your calf against the horse's side. Your toes should only slightly pointed outwards, and your knee should be against the saddle. But, when I say against the saddle, I don't mean pinching. Just slightly against it. Also, your hands and arms should be nice and quiet, not flopping around.
Lean forwards slightly more, and that way your upper body can be quieter.
When cantering: You're actually doing pretty good at cantering. Your legs are nice and quiet, and it seems that your toes are better. Just remember: heels down. Another thing I noticed is that you are off balance. You are leaning to the left. This causes your horse to be off balance while cantering. Think of putting equal weight in both stirrups.
When jumping: At one of the jumps your leg wobbled a bit. You also seem to get into 2-point position a little early. However you are not jumping ahead, and that's good. On a few of the jumps you came back a little late, and stay in jumping position longer than you should. Is that intentional?
Other than that, you are a very good rider. Please don't be offended by my critiquing; I actually do think that you are a great rider and those are the little errors that I see. Good job. :)
Anyways... *hides in box incase my critiquing was wrong* :)
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