The Talmud relates that Rabbi Prayda had a student who needed to hear a lesson 400 times until he comprehended it. Rabbi Moshe Rosenstein used to say that when it comes to elevating our attitudes, we are all the same as that student. If we sincerely want to integrate a new attitude, we need to repeat it over and over again. Even if a person is highly intelligent, unless he reviews the concepts hundreds of times, they will not be properly ingrained.
(see Daas Chochmah Umussar, vol.1, p.114; Darkai Mussar, p.60; Gateway to Happiness, p.69)