The Rambam (Hilchos Teshuvah 2:2) states, “What is teshuvah? It is (1) to leave the sin, (2) to remove it from his thoughts, and (3) to be determined never to commit this sin again.”

In other words, teshuvah is a decision. When one decides that he won’t commit this sin anymore, this is teshuvah, and that conclusion can be made in a moment. ~ R’ Elimelech Biderman