Besides your specific area of study, there is much to learn in grad school. In a phrase, I'll call it self-governance.
There are several components to self-governance. Firstly, no one is going to buy the groceries, pack the lunches, cook the dinners, clean the clothes except you. These things should come before school. Without hearty meals and non-grimy clothing, you will surely wither into a greasy, malnourished loser. These necessities form a foundation. If they crumble, then the whole building crumbles. So, health comes first.
If it appears that you have a free afternoon, then you are mistaken. There is always something that needs to be done, whether it is for school, a potential job opportunity, or just a nagging errand. Every minute matters. Simultaneously, one must consider their sanity. No one can just work and run errands every waking moment. But if you want a break to enjoy the outdoors, exercise, nap, or socialize, then you must plan it in to your schedule. Simultaneously, again, one must accept that the schedule never unfolds as initially planned. Every day morphs, and you have to adapt.
Thus, the goal is a kind of flexible rigidity--a kind of balance.
That's a glimpse of the overarching picture of self-governance. On the psychological side, I think the goal is also a kind of balance. I see it as two parts: Confidence and Substance. By Confidence, I am referring to the courage and belief that one can and will succeed (Whether you think you can or you can't, you're usually right. -Henry Ford). By Substance, I am referring to the base of knowledge accrued through earnest study habits.
Since the apparent goal of professors is to assign more work than is possible to finish and to challenge grad students beyond their capabilities, the mean level of student Confidence is often below the detection limit. Some colleagues think they can build Confidence through hard work and long periods of study, through Substance. This is true, but the resulting Confidence boost is small and temporary, only until the next big hard assignment. Other colleagues think they can build Substance through unjustified levels of Confidence. Remarkably, this is also true, but tenuous. The best balance is to develop a strong belief in yourself, even if there is not much backing to hold that strong belief, and then to study like mad to justify that strong belief, accepting the fact it will never be completely justified.
Substance builds Confidence, Confidence builds Substance. You need both.
Revisiting how professors assign more work than is possible to finish, new grad students are forced to make a mental transition. They must switch from their self-imposed standard of work quality (which is probably what got them admitted to grad school) to a new standard of quantity. Yes, this is terrible for those of us who would rather turn one perfect assignment than twenty good assignments. But when you think about it, the latter achievement is more impressive, and more efficient.
The best way for me to explain is to reflect about a ceramics course I took as an undergrad. Our assignment was, given a fixed amount of clay and twenty minutes, to build the tallest sculpture we could using the coiling technique. Everyone worked with haste, and everyone made a respectable piece. Critiquing our accomplishments afterwards, the consensus was that imperfect-yet-perfectly-adequate work could be achieved in a short time. It's a lesson that's reinforced in graduate school.
One last point about self-governance. Planning ahead takes time and energy. So if you happen to have time, energy, and a completeable task in front of you, then do it right away instead of using that time and energy to make a plan to do it later.
To sum up:
-Health comes first
-Every minute matters
-Keep a structured schedule but be willing to deviate from it
-Plan in time for fun stuff
-Be confident, build substance
-Realize that good work can be completed in a short time
-If you have something to get done, do it right away
To govern oneself, to employ these lessons, that's what you learn in grad school.