It might help if you use a corner (I used to have similar issues with Cinny). Basically you want them to face the end of the arena whth a side rail going along your horses side. Then you Sandwich them in with the mounting block. They can only move backwards. Try to think of making a mini pen with the rails of the arena and your mounting block.
Then, patience. Let your horse know it is absolutely unacceptable to move. I used to back Cinny halfway across the arena quickly every time he would move while mounting. Then we would go back to the block and try again. After a while they start figuring out that it's more work to NOT stand still.
Just remember patience patience patience. It' best when you aren't in a rush to get on your horse. Try this out when you have time to kill, or go early so that you can create time to kill with this.