Let's go back to speedy horses..... being able to slow down the speed of your seat bones and slow down the speed of the horse's legs is the key thing here. If the horse moves your seat bones at his speed, then 'he takes you', when you slow them down to the speed that you want and are in control of, then 'you take him'.
Another big factor is breathing. You may well find yourself holding your breath on a horse that speeds off with you - and of course, he may be holding his breath too! It works well to think of 'breathing for both of you' and 'breathing down into the pony'. This may sound like kids stuff, but I have helped one international Grand Prix Dressage rider make huge changes to her GP horse by breathing down into him! It totally changed his body quality, making him much softer and less brittle. Think of the breath as a colour that goes down through your body and into his.
Put your breathing into a rhythm with the horse's walk. Count his forelegs moving, and breathe in 2,3,4, out,2,3,4. Maybe you can do 6's. Make sure you think of the breath going down - many riders are upper chest breathers, who lift the front of their ribs with each inbreath. Try not to do this: your ribs can expand outward, but must not go upwards. That automatically brings the breath more down, and you can amplify this effect by imagining that the breath is drawn down into your pelvis.
Breathing hold the key to so much, and I will come back to it in future posts. If you are coming up to something scarey, deliberately breathe OUT - we tend to breathe in and then hold our breath. If you make yourself breathe out you can stop that from happening, and you automaticaly send a much more calming message to your horse.
All the best!
Mary