I don't know anything about your horse, but when I got my horse he had been a very well trained school horse, lol and did the same thing. ( because the students let him. Lol) So what I did and worked very well , even though he loves to work<If he tried to move while I was mounting him, I made him go into a trot right away while I was on the ground. If he tried to move as I was getting on then he had to work. Just 1 or 2 circles, but it made him think twice about moving while I was getting on. I also used the "stand" command and since he is only ridden by me now and he is no longer a school horse the consistency has definitely proved to work after only a few times. Now my boy knows what is expected of him at all times. (lol) I hope this isn't taken the wrong way towards school horses or students! That is not how I want it taken, Just that sometimes you need to make them work more if you want them to not work. I hope that makes sense.
Sorry if not.
I am owned by the Sandman