Your horse is so pretty, and trying so hard, but he appears to be significantly lame in his hind end. He is very unhappy about taking a lope, and his hind leg movement is short, choppy and uneven. At first I thought it was his right rear, but I'm now not sure which, or , BOTH hind legs are very obviously 'off'. I think it is either something in the Sacro illiac area, or the stifle, and not sure , as I said, if it's one or both rear legs.
Has this horse been evaluated by a vet?
First you get with the horse, then you get the horse with you.