If it makes you feel better, I had the same issue in my first module (biochem genetics etc). I got a B even though I worked like a mad person. In my second year now, and I haven't gotten a B since =)-or even close.
I just learned how to study (prereading, NO LAPTOP IF YOU GO TO CLASS, rewriting notes, making lists of buzzwords for the insane details and doing a 100 or so questions the day before the test).
Just because you are struggling now doesn't mean you will always. Don't put so much pressure on yourself, don't compare yourself to everyone else, and don't think you are stupid if you have trouble. You don't have to get As-you have to be competent and sometimes the crap on your test isn't relevant at all to what will make you a competent doctor/intern/resident. IE I had a question on a test today about how to treat sore nipples from breastfeeding-gah.