Why all the negative comments?
To the OP: a B+ is relative. If everyone else is getting B's too then it's OK. USMLE is more important than class grades, anyways. If you've been working hard so far and you study hard for Step 1 then you can match a good residency. Class rank isn't the most important thing by far: clinical rotations and Step 1 are better indicators of your ability to match somewhere good.
Find the source of the problem and correct it. If your school offers counseling services, go see them and they can give you good tips on how to do better.