My grad-school situation was a little different, since I was working full-time and taking classes part-time. But I did face a similar dilemma of how to know when I was doing enough and how not to feel guilty when doing anything non-school-related, and one thing that helped me immensely was to set aside times when I explicitly allowed myself not to study or do school stuff. I gave myself Friday nights and usually either Saturday or Sunday afternoon or evening; for you, it might work better to set aside one or two hours every evening -- "I don't do school stuff between 8 and 9 pm" or "after 9 pm ever" or something. No guilt allowed. I was pretty vigilant about sticking to this, too -- except during the last couple weeks of the semester, when all hell breaks loose and there's just nothing you can do about it.

Having that "no guilt allowed" time helped, but what also helped about it was that if I felt tempted to goof off at other times I could say to myself "stick to it tonight and then tomorrow night you can relax" --having that time to look forward to helped me push on through at other times.

I have no idea whether that helps you at all or not. You'll figure out how to make it work, though!