We talk about confidence a lot in the horse world. If a horse does not have a confident leader, he feels he needs to take charge, and that can lead to all sorts of frustration for his human partner. I find, in general, confidence is not based on knowledge. People either walk into their first lesson with me "knowing they can do this" or "hoping they can do this" and their previous experience has a very small amount to do with which side they land on.
The "hoping they can do this" folks are usually the ones who want to succeed the most. They are also usually the ones who have a hard time. Different horses respond differently to a lack of confidence. They may be inattentive to their handler on the ground, wiggly, or hard to steer. No matter how it manifests, these horses are not being bad, and their training isn't ruined; they are just responding to their leader.
Unconfidence can be caused by fear, lack of familiarity, or a past experience. It's a terrible roadblock to learning more about horses and can be difficult to overcome. So, how can we build confidence? We just fake it. Figure out a realistic goal for your ride and make an action plan if your horse does not do what you ask. Having plan A and plan B puts purpose into your actions. Your horse will definitely notice this. Often, this is all it takes to bring a horse with a mind of his own over to your way of thinking. The more successes you have, the more confidence you will build. The key is to take it slow, pick realistic goals, and stick with it!