As a university student, you are no longer forced to stay in school all day. Apart from your class time, you organize your own time and place for studying. You are there to study (an active process) rather than to be taught (passive). Your teachers may or may not urge you but they do want you to share their interest in the subject and the want you to succeed. The teacher works as your coach or trainer. If you were to become a professional tennis player or musician, you would expect your coach to look out for ways of improving your performance, and to criticize your work all the time. You would be silly to feel fed up about this. In fact, you would sack your coach if he or she just said everything was fine. So if you get work back with a poorer mark than you expected and with various criticisms, don't feel discouraged by this; if you don’t understand what is wrong and how to improve, then ask your teacher to explain and don't be satisfied until you do understand.