I've always had confidence/assertiveness issues with horses. I'm a very non-confrontational person, so being around horses has really forced me to leave my comfort zone sometimes.
To build up your confidence, be around quiet, safe horses as much as you can. Spend as much time grooming and leading (handling from the ground) as you can. Find out where your problem areas are (picking up feet, leading into stall or pastures, etc.) and slowly work on them until you feel really comfortable.
Have horsey people around you when you work with them, and don't be afraid to ask question after question :-D "What makes him do that?" "When he stomps his foot like that, what does that mean?" "What's the best way to...?" You'll get lots of answers if you ask lots of questions.
If at any point you feel really uncomfortable then let a more experienced person know, let them handle the situation, and ask them what you should do if the situation arises again.
A big thing that helps my confidence is knowing what to do and have a plan going into a situation where a horse doesn't behave perfectly. Learn how to handle spooking, bucking, rearing, learn how to do an emergency dismount and emergency stop. If someday you walk by a terrifying plastic bag blown by the wind you'll have tools in your toolbox to handle it.
A lot of people had said this but fake it until you feel it. Ask the horse nicely but firmly for what you want, but be prepared to TELL them what you want if you don't get it right away. Never rationalize, "That's good enough." If you want the horse to move over two steps, don't only go one, don't go three.
So top four things - spend as much time with horses as possible, learn how to handle emergency situations, and fake confidence until it's real, and don't settle for less than you asked for!
hope this helps