Does your excitement or energy get the better of you when you are mounting your horse?
"Horses pick up on and are very sensitive to our energy. If you are struggling to get your horse to stand still, try lowering your energy and focus on what you want (not what you don't want). f I said to you "don't move" your mind thinks move. If I said "stay still" your mind thinks still. So think or speak "Stand still"
Practice not getting on! stand on the mounting block and chill out, your horse will learn to mirror you