The education of a horse is not a causal undertaking nor is it for the fainthearted.
Your greatest responsibility is to have the horse trust you. Trust means not being afraid. It is the easiest thing to lose, it is difficult to regain once lost, and even if you do get it back it will not be as good as it could have been at the beginning.
Use of force or abuse begets resistance and fear.
Work with the horse from where he is at, not from where you want him to be.
It is not you (the teacher) who tells the student (the horse) that he has learned the lesson. On the contrary, it is the student who tells the teacher that the lesson has been learned.
Observe what the horse is telling you he needs by listening to the horse. For instance, when working from the ground look to see if the horse is traveling relaxed and fluidly or is his body exhibiting tension by moving mechanically with choppy steps with his head held high and his neck stiff. Look at his ears: are they pinned back, look at his eyes: are they wide open and wary, look at his muzzle: are the lips tight with wrinkles around them, look at his tail: is it clamped tight against his buttocks. All of these things and more are indications of tension.
When riding him, feel if he has tension or stiffness in his body by bracing his head and neck when asking him to stop or pushing his nose against the bit, resisting your efforts, is sluggish when you apply your cues or refuses to comply to any request you ask of him. These actions are just some of the ways the horse is telling you that he does not have a clear understanding of what you want him to do.
Explain what you want the horse to do in a way that he will understand it.