Horses are highly perceptive animals who learn faster than humans do. They have to learn fast to survive as prey animals. In our human effort to communicate with the horse, we tend to forget to take advantage of their quick learning abilities. In our rush to fulfill our agenda, we go too quickly and sometimes impatiently, through force (e.g., heavier bits and leverage reins) which causes the horse to engage his survival instincts, rather than his intellect. When the horse is in survival mode, there is no lasting learning effect on the horse. He is merely trying to live for another day; there is no cooperation or communication happening.
So what to do? Learn to call on their intelligence and quick learning abilities in our interactions with them. Horses love to learn, and relate best to clear patterns that can be broken into clear parts. They understand intentions and are masters at reading body language; connecting the dots is their superpower. So we must slow ourselves down and learn how to break our ideas into smaller dots that the horse can enjoy connecting.
If we can break our ideas down into smaller and smaller pieces for the horse he will connect the dots on his own. Try slowing things down to a walk. Do your training in the walk first and evaluate how well your horse is getting what it is you are teaching. If he is having trouble, give him simpler tasks (or easier dots) to interpret. When you are sure he understands the lesson, then try it at the trot. When we train at the walk first, it gives us the time to let the horse soak on a particular part of a lesson he may be having trouble understanding. Also, we can work on ourselves, making sure all our aids are working softly. If you develop the skill of slowing down and making smaller, simpler dots for the horse to connect when needed, you will find you do not need to repeat lessons, and your training will actually go faster.