"It is not expected of you to complete the entire work" (Pirkei Avos 2:21).
Frequently, a person is discouraged largely because he has goals that are beyond his ability to reach. While we do have much more potential than we utilize, we still are limited in what we can do. We must set reasonable goals. Our sages tell us that we are not expected to complete the work. There is much Torah to study and many good deeds to fulfill.
A person who demands too much from himself will feel frustrated and discouraged and is likely to quit. Therefore it is important to be aware of your limitations. But some people might feel that if they cannot obtain perfection and cannot complete everything necessary to be done, they might as well not even try. In the same Mishnah the Sages state, "But you are not free to refrain from the matter."
Choose the middle path. Try to accomplish as much as possible, but also realize that you will not be able to do everything. Learn to take pleasure in trying to accomplish without feeling frustrated from not being able to finish all that you would have wanted.
(Gateway to Happiness, p. 380)