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.