The video camera is your friend, here. If you don't have one then borrow one. Or rent one. Or buy one. But get someone to video you as you ride and as you are being coached. Make sure that they know how to run a video camera!!!!!!!!!!!!!!!!!
But be prepared for what the camera will show for the camera lens does not lie. It can be manipulated if the human chooses to do so, but if the filming is honest then what you see will be what really happened. The lens does not care who you are or where you came from or how much you paid for a horse. That makes it a true friend, but one that you might not want around all the time.