I would suggest using a chant not a spell, spells wont help you gain confidence they give you confidence for a short while but you never get to hold on to it. A chant will help ease you when you are in a situation and will effect your subconscious to the point you'll start to believe in yourself and you'll eventually start living with confidence ever day regardless if you say the chant or not.
Magickal-Quests chant page has several chants for different purposes there's one at the bottom for confidence.
Or carry a charm bag, Startwitch (admin and owner here) has a charm bag for confidence on the main page of the site here's the link :
http://www.everythingunderthemoon.net/s ... ations.htm