I don't think I saw your year old critique request. But, the thing that just pops out is that your arms are as stiff as boards. really , really straight and stiff. There is no movement or give in your elbow and your shoulders are locked forward, really closing off your chest. You could not take much of a deep breath in that position. You were in a half seat at the canter, right? it just looked so very stiff. You are in a position that if the horse stopped suddenly, I think you might go up on her neck. There is not core to center yourself around.
So, you should really get those elbows back some and roll your shoulder back. Your instructor mentioned it and she is right.
REally cute pony. The draw reins? why are they on?