Self-esteem can't win you a race if you're not in shape. - Louis Zamperini (former World Record holder for the mile run)

 There is no substitute for practice and preparation. Every athlete knows this. It also applies to nearly every other field of human endeavor. Musicians spend far more time practicing than performing. Airplane pilots spend enough time training in simulators that when an in-flight crisis occurs they will respond quickly and accurately.

The successes that result from thorough preparation is what produces positive self-esteem and self-confidence. Not the other way around.