I heard someone say once to get a horse to lope slowly on a loose rein you have to lope a lot, and when the horse decides to lope a circle at the speed you want, quit. We do the same thing with collecting, or any "issue" we're having. The horse is done working when they've stayed collected and in the bridle for 2 laps around the arena, or have performed whatever maneuver we're working on successfully. Quit when they do what you want. Oddly enough, the horses are usually quick enough to catch on.