Try to put its head by the wall of the arena or use voice commands
As someone who owns an OTTB, I would say the best method I found is transitions, transitions, transitions! :) I would start at the walk, and once you get a nice walk go to trot. If your horse speeds off, go back to walk. Repeat until you get the trot you want. After, transition to canter. If you dont get a nice canter, go to trot but if you get a speedy trot go all the way back down to walk. Repeat until you get the canter you want. I would recommend doing the walk-trot exercises for one ride and then the walk-trot-canter once you're confident with the walk-trot exercise. Hope this helped :)
If they start going too fast stop and back up. They will eventually learn that slow is better than fast and back. If it is a gelding good luck they are stupid
If they go to fast, make sure to give and take. Pull back some, then give them some rein. If you just keep pulling they will get even faster to get away from your pulling.
Transitions transitions transitions.
For flat work, definitely transitions, circles, serpentines, poles, leg yields using half halts, and general turning work :) Good luck!
For jumping definitely poles! Put 'placement poles' between jumps, or even make them into bounces. Bounces make sure the horse keeps a steady rhythm and works great. For flatworm if you horse speeds up, do a tight circle everytime speed increases without cue