This place is awesome. As "only" a rec ball coach, and the daughter of two rec ball players, I get a ton of ideas all the time. Just wish I could find the one sure bet on how to teach confidence to a kid who has the ability but doesn't realize it, but I know, this isn't discuss childpsychology.com
One of the things you can do to help a player build confidence is let her know it's ok to fail as she works on her game. Often players who lack confidence are afraid of failing or disappointing their coaches or parents. They let fear of what might happen keep them from really going for it.
Encourage this type of player to focus on the process - what they're doing - instead of on the outcome. It can make a huge difference.