Mastery over Self is the most important factor to help one's success in life, as per the following sayings and quotes.

Though one man conquer a thousand times a thousand men in battle, he who conquers himself is the greatest warrior. - The Buddha.

He who knows others is learned; he who knows himself is wise. - Lao-tse.

He is strong who conquers others; he who conquers himself is stronger. - Unknown.

No man is free who is not master of himself. - Epictetus.