Back to speedy horses...
It always pays to be on control of the speed of the legs in the slower gaits before you go to a faster one - so do not trot until you controlling the speed of the legs in walk, and do not canter until you are controlling them in trot. Rising trot becomes an important skill here, and the first way to think about slowing the legs is to think of making a pause in the saddle as you land, and a pause in the air at the top of the rise. Each of these pauses happens as one diagonal pair of legs is on the ground, and it helps to keep them on the ground for longer, becoming the antidote for the horse who is like a 'cat on hot bricks'!
That sounds great in theory, but most people arrive in the saddle only to find that they are instantly catapulted out of it! This will inevitably happen if you are hollow backed, and land with your seat bones pointing back. You will also hit trouble if you forget to rise because you are thinking so much about the pause. The trust you make as you rise must match the thrust of the horse's hind leg, and this can be an act of faith on the speedy horse. All your instincts (especially if you are a nervous rider) will direct you to barely leave the saddle. But if you do this you are 'left behind' in the rise, becoming the water skiier to the horse's motor boat! Then, as he 'pulls the rug out from under your feet' your plight can only get worse. it is absolutely critical that your centre of gravity 'keeps up' with his.
Think of the windscreen wipers on a car. They always get to the top of the windscreen, and potentially they could pause at the top and pause at the bottom without this affecting the size of their 'wipe'. If you manage to make the 'full wipe' and the pause both work together you will know - even if you only manage the pause on one sit. It is inescapably different, and the horse's response is instantaneous.
This is a hard skill to 'get' on your own, however, and you will be doing really well if you can translate my words into action without any outside help. I hope you can!