Catechism of the Catholic Church §2342 — excerpt
Self-mastery is a long and exacting work. One can never consider it acquired once and for all. 128 The effort required can be more intense in certain periods, such as when the personality is being formed during childhood and adolescence.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.