I think for a horse to be a brave leader you have to be one yourself.Most horses will only be calm when your calm.
if you are scared or tense, the horse may know it before you do. that tells them that there may be something scary up ahead. if you are calm, the horse can sense that to and trusts you to lead him forward.that is my experience with horses.
I know that I tense up even if I try my best not to, when riding on the road and and meeting a car. Because of a bad experience meeting a car once before.
Also, is it possible you are petting your horse when he balks or spooks, thus inadvertently rewarding and reinforcing his negative behaviors? It's easy to think we're being encouraging and reassuring without recognizing that the reward needs to happen after the horse finds the right solution which is to pass the object calmly.