Confidence when in the presence of a horse is the MOST important tool any horseperson can possess. You need to take a deep breath, let all the butterflies in your tummy escape, and tell yourself you are in control and will remain calm no matter what happens. A horse will not listen to you if you are afraid. Horses do not follow fear, they follow confidence. So start by doing things you already feel confident in and then baby step your way up, and your confidence will naturally grow.
I think that people spend so much money on horse "stuff" to better control thier animals when the thing they need most is confidence. You take a confident rider and they can do some pretty amazing stuff on almost any type of horse. You take an unconfident rider and put them on a bombproof horse and they may still have issues. Horses spend thier lives responding to confidence/dominance. It is as much of a natural instinct as the flight drive in a horse. It will respond to confidence and it will follow it. It will look for it instinctively.
My daughters pony is a very cocky little brat with my daughter because she knows my daughter is afraid. I have instructed her to not be a passenger in the saddle, she must believe she is the one in control, to be the one in control. The pony will go through a few minutes of stupidity whenever she gets on her, she trys everything bucking, running back to the gate, rubbing her off, she has even gone so far as to get down on her front knees. BUT if you stick it out and show her she can not scare you away she shapes up and acts proper. When I get on her she is just fine, only with my daughter who is afraid, does this pony try this crap. It is the same when you send a horse off to the trainer because you are afraid of your horse. The trainer who has lots of confidence from all of the experience they have with horses usually has less issues with certain behavior, ITS ALL ABOUT CONFIDENCE.
When dealing with horses things do happen. A horse can rear up, it can buck, it can kick, bite and do all sorts of nasty behavior and you need to educate yourself to understand what to do if it does happen and how to predict when it is about too happen so that you can find assurance in being prepared. It will help to calm your nerves a bit if you better understand what is going on and how to deal with it. It gives you a sense of control when you educate yourself.
I know this is sort of long, but you need to build up that confidence, but stay relaxed too. Always be relaxed and confident when dealing with horses. Take a deep breath and tell yourself "I Can" instead of "I Can't"