One of the quickest ways to gain confidence is to pretend you're confident already. It may seem silly at first, but if you begin to move your body and speak like you would if you were confident, you'll quickly gain self-confidence.
How fast does a confident person speak?
At what volume does he speak?
How does he gesture with his hands?
How fast does he walk?
How does he move his body?
Pretend you're confident by moving your body in confident ways. Your motions will follow the movement of your body.