A recent study of about 5,000 science graduate students from a dozen countries revealed that students tend to become more displeased as their degrees progressed with 76% of first year students reporting being satisfied or very satisfied to 67% and 61% of second and third year students, respectively. Why? One theory is that as students advance they become more independent and their focus shifts to their own research, which is anxiety provoking.
Another view is that advisers may not provide the guidance that students seek. Students perspectives of their relationships with advisers declines along with overall satisfaction. About 63% of first year students were very or somewhat satisfied with the guidance that they received from their advisers, dropping to 53% among second year and 46% among third year students. Of fourth year students and beyond, who are likely working only on their dissertations, only about 43% report satisfaction.
Of course, all students experience some dissatisfaction once the reality of graduate school sets in. But help yourself in getting what you need by choosing an adviser that fits and developing a good relationship with faculty. Know what you can realistically expect of your mentor. Also, don't make your work your life. Take time off for yourself - time away can help you recharge. Finally, be self aware. If you're truly miserable may you need a break, perhaps a permanent one.