I came across a short list of sayings from some of the most influential of the ba'alei ha-mussar (I almost wrote the greatest of the ba'alei ha-mussar, but the nature of such people is that the greatest were probably anonymous and lost to history). Rather than copy the entire page here, I picked only two quotes. Go there for more:
"Man wants to achieve greatness overnight, and he wants to sleep well that night too."
- Rabbi Yosef Yozel Horwitz, Alter of Novarodok
Rav Itzele (Peterburger) Blazer, a disciple of Rabbi Israel Salanter, says that there are two books in heaven, one of sins where you gave a sigh when you sinned and one when you didn't, and the difference in punishment between them is greater than the distance between heaven and earth.