How to Be More Confident?

To some degree, confidence is the key element to success. If you be more confident, you can get a higher score at school. If you be more confident, you can get more trust from your boss and colleagues. If you be more confident, you can attract more suitors.

Sometimes, it seems that I am a girl with enough confidence. But sometimes, I am just a  coward. I don't know how to express my feeling at this moment. The only thing I can do is working harder and let it be.