It is really hard to ride nervous horses. They are not calm and they might not be quiet to be mounted. They may spook at the slightest noise around making it all difficult and frustrating to keep it under control.
There are also riders out there that want to fight fire with fire and what they do is that they try to force the nervous horse to obey by using stronger bits, draw reins, leverage rigs, spinning the horse in tight circles, withholding food and water, or by tying the horse for hours so “he can think about what he did wrong.” Most riders do not have the patience or the skill to deal with nervous horses and usually, they do more damage than being helpful. If you are in the situation of having to deal with a highly nervous horse the logical first step is to try to determine the source of the nervousness. Is it too confined? Is it too high? Is it ridden too little? Is he afraid? If afraid, afraid of what? Is he green? Only when you get to know the source of nervousness can you start to practice some possible actions that can really benefit the horse. Horses are not people and they cannot speak up. Get to know them, love, and respect them, be there when they need you and then you will feel the difference in behavior. Until you can fully understand that truth, you are not yet ready to train horses.