I don't have advice, so much as just some support.  You are not alone.
MS1 here with little motivation to work harder than I am.  Consistently a B student, but progressively working less and less each test.  I'm content with my grades, but it does make me feel pretty guilty when I don't pick up the books.  Not sure what to do about it, except hope that I have better motivation in year 2.
However, unlike you, I do enjoy other things outside of studying.  Have a variety of hobbies, a sweet girlfriend, and an xbox... more than enough things to distract me.