Before we do anything, we should we always ask ourselves whether we will be able to do it properly and complete it. If the answer is no, we should not start. Leaving tasks uncompleted creates a habit for the future. So once we have begun something, we should be sure not to go back on our decision.
Self-confidence is not to be confused with pride. Pride is thinking highly of oneself without good reason. Self-confidence is knowing that one has the ability to do something properly and being determined not to give up.
Ordinary beings are prepared to make a good deal of effort for relatively insignificant ends. We have promised to work for the immensely more important goal of liberating all beings, so we should cultivate great self-confidence, thinking, even if I am the only one to do so, I will benefit all beings.
-The Dalai Lama, A Flash of Lightening in the Dark of Night.