Well I was watching xxxHolic, and happened to come to an interesting phrases.

"Words have power over our roles in our lives"

Let me interpret it. It means, we humans are restricted by those words.

As an example, we'll never prevail if we keep telling ourselves, "I can't do it" / "I will fail" / etc. Thus, before even we tried doing that particular thing, we'll ended up in failure.

Then, the famous quote came into play. "We mustn't give up before we tried."

Got it? That's why for those who are still sulking and despairing due to your lack of self-confidence and keep telling yourself can't do it, just grant yourself the courage. Nothing comes without a failure; and you'll be a definite failure if you didn't attempt.

Losing/Failing means nothing but experiences, get over with it and give up your stubbornness. ;P Things will work out somehow, believe it.