Horses need experience. When they are green and you start riding them out of an arena, you need to ensure that you don't put them in dangerous situations. The more they get ridden and see that you are a confident leader who doesn't put them into a dangerous situation, the more they trust you.
That said, when it comes down to it, if they see something that truly scares them, they can still react like the horse they are. When that happens, you need them to be trained enough to be able to disengage them and get their attention back to you.
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