A person cannot break his inborn personality. But everyone has free will to choose how he will act within the basic structure of his personality. Whether you will be righteous, evil, or average - it's up to you!
As the Talmud (Shabbos 56a) states: A person born with a tendency to shed blood makes his own choice as to whether he will draw blood for healing, or as a robber, or as a butcher, or as a mohel (ritual circumciser). That is, the choice of how to express your basic personality is the key factor in determining your greatness (or lack thereof).
King Solomon (Proverbs 22:6) instructs us to "train children according to their individual natures." When your approach is in alignment with the child's personality, he will continue in this positive path his entire life. If, however, you try to force the child (or anyone, for that matter) to act in a way that is inconsistent with his basic nature, he might listen superficially out of fear. But as soon as he can escape your authority, he will turn away from what you have taught him.
This concept is so valuable, that it is worth reading it five times - to internalize and remember it.
(see Vilna Gaon - Proverbs 22:6; Rabbi Pliskin - "Consulting the Wise")