I couldn't think of anything to add that hasn't already been said. I agree; once in awhile, let him go .
And, use the cirlce to slow him. Think of it kind of as peeling off. You have a larger cicle that you are working on (the whole arena, perhaps) , and when he starts rushing, you just make the circle a lot smaller and wait for him to ease up. When you feel him change in speed and attitude, allow him to come out of the circle and trot forward in whatever direction you are facing, no matter if it's a continuation of the direction you were going before. Just open the door and let him choose the speed, I mean let him choose to go the easy trot that you are asking, and if he chooses to speed up, you peel off into the smaller circle until he chooses to slow down, then straighten out and go forward. Again and agina.