I used to feel the exact same way that you did; if I wasn't in class/clinical, then I felt like I had to be studying and if I wasn't, I was doing something wrong. And yes, I did well on my exams, but after a few classes it just became too much. There is already so much pressure built into nursing school
that you don't need to pile more on yourself.
As with most things, studying should be about quality, not quantity. For me, I can go 1-2 hours max before I need to take a little break. Doing something else, even for just a few minutes can help you unwind and clear your mind a little bit.
What helps me is to make sort of a "study schedule." Based on when the exam is, and how much information will be on it, I decide ahead of time what topics I will study and really get down pat that day. It helps me not to get overwhelmed. Also, it helps with the guilty feeling of not getting enough done, since you accomplished all that you set out to do.
We are not machines and we cannot just go on autopilot for hours and hours at a time. It's not healthy and will only lead to burnout and your hating nursing school. Taking care of yourself and giving yourself time to breathe is never anything to feel guilty about. Good luck with studying!