Circles, and lots of them, are a great way to get a horse to slow down without being on their mouth the whole time.
Anytime that Flynn starts to go faster than you want, turn him into a small circle. You can either direct rein or neck rein. I personally like to direct rein and ask them to give me their nose at the same time. When he's in that small circle, he'll have to slow down his trot. Now, don't make the circle SO small that he has to break down to a walk. But small enough that it makes him slow down.
Circle until he is paying attention to you and then allow him to move on straightaway. If he speeds up again, repeat. You may have to repeat this A LOT until he starts to understand that it is simply easier to trot slow, and you won't make him do a circle.
Of course, circle both ways randomly, so you aren't always circling the same direction.
I like circling better than "checking" the horse, because (if you direct rein) you are only engaging one side of the bit, and not both, and thus staying out of their mouth more.
Serpentines can also work wonders too, so you can mix those in there too. And again, you can simply use one direct rein at a time, so you are staying out of his mouth.
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