Help me slow this guy down...

The big thing is to get the horse to think of something else than running or slowing down, give it something to do that demands some effort and/or thought will usually slow them down.

Much like you did when you walked and turned etc without much rein contact :) The horse had to listen to the seat and try to figure that out, instead of feeling the bit in it's mouth and work itself up and start to stress :)

If you have a horse that's fizzy and just wants to go look at what your feeding it oats and barley is one of the main problems cut out them and feed the horse on cool mix with out any oats,barley,corn and wheat and put a hand full of apple chaff in the feed and you will see a difference in a couple of weeks but still give the horse a bit lunge every day and you will have a totaly different horse