I know its dissapointing to get a grade lower than you are used to. I don't want to be dismissive of how you feel. It makes total sense to be anxious about something you are working so hard toward achieving. But being 10 years out of undergrad or so, and a recovering perfectionist, I can assure you getting a B in one history class will not have ANY impact on you at all as you move forward, even if it is disappointing now. You've got a stellar GPA and MCAT. Maybe if you can't get the hours up before you apply, try to get some help with your personal statement and how you are narrating your application. Sometimes its just about finding your voice properly to demonstrate the significance of the experiences you DO HAVE, rather than dwelling on what you don't have, or wish you had more of. Try to separate how you're talking to yourself from being too hyper critical. It isn't a very motivating force to focus on all your perceived flaws (this is hard to do when you're worried, I know, but beating yourself up won't make anything better).