I read an interesting article by Hilda Gurney that had been reprinted from the 1970s. In it, she explained that if a horse ignores your half halt, halt him all the way several times so that he learns to listen to the half halt. I did two or three of those, and suddenly I had a more forward, energetic horse that was (mostly) even between the reins.
I haven't done this yet as I am always too busy praising him for the first one to even think about asking for another one. Even so, we circled around and crossed the diagonal again where I had to tell him to wait for the new canter aid. He gets pretty ramped up about it and wants to just get it done as soon as he thinks that's what I want.
Even with a bit of rushing, he still gave me a clean change that didn't involve a bolt or a buck. I glanced down at my watch and realized that in only 15 minutes, Speedy had far exceeded what I wanted to accomplish that day. I hopped off of him, and slipped him some candies. He knew I was proud of him.