Kevin1990--
I recommend that instead of stressing out about the grades you've earned, sit down with a copy of your transcript and your course materials and spend a few hours figuring out how/why you got the grades you received. The objectives of this exercise are two fold. First, you want to make sure that you understand the core themes and lessons of a given class. If you do understand a course's main points, that B+ is going to serve you a lot better than you now realize.
Second, you want to identify your strengths and weaknesses as a student and to identify ways to improve the former while shoring up the latter. During this exercise, see where you might have missed opportunities to do better and give yourself credit for exceeding expectations.
In some cases, you might decide that you are who you are as a student For example, you might not do as well in lecture courses where the professor talks to the whiteboard and expects undergraduates to write down everything he says and then do a memory dump in blue books. Or you might just prefer to wait to the next to last instant to write an essay.
In other situations, you might decide to adjust the way you approach certain types of tasks. You might conclude that listening to music while reading decreases your focus more than you previously thought. You might determine that you benefit from making flash cards for the identification portion of exams. You might find yourself benefiting from spending more time with professors during their office hours. (You might decide to trust your instinct and drop a class if you realize you won't develop good rapport with your professor.)
More generally, I think that if you are going to walk the path of a historian, one of the best ways to advance your candidacy to graduate programs is to develop answers to questions like "Why should professional academic historians study war?" and "Why should military historians study professional academic history?" To be clear, you do not have to develop an argument that military history is the most important field of history. (IMO, it isn't--at least not in the study of American history.) But prepare yourself to participate intensely in conversations with, for example, social historians so that they may reconsider long held views on the relevance of war to their research interests.
From there, start developing your views on which aspect of warfare is the most vital. From your posts on this BB, I think your answer is "strategy." But why? Why not grand strategy or operations or tactics or logistics or C3I or civil-military relations or technology or special operations or any of the other many sub fields. This exercise will start you on the road of figuring out how your tree fits into the thicket and how that thicket fits into the larger forest (of military history) and where that forest is on the broader historiographic landscape (i.e. the impact of warfare on the history of region X during interval Y).