Hey! If you wanna slow down your horse just stay very calm, lift your hands a little and get your whole body to kind of lean back. Make sure you push your feet forward, but not too much and just stay consistent don’t just pull all at once make sure you do it gradually.
If you have an arena at home practice doing lots of downwards transitions on the centre line. So trot-halt, or even just walk to halt so you have them listening. Then you can practice canter - trot / canter - walk. Also turning off the centre line at x to do 10m circles will keep your horse attentive and not teach them to rush in a straight line. Hope this helps :)