Our son had similar advice from his admission officer as Caitmarie had. Please remember that USMA, as well as your sources for nominations, are looking at the balance of the "Whole Candidate", which takes academics, physical fitness, AND leadership potential all into consideration.
Grades are a part of the academic assessment, but so are your ACT/SAT scores. Take these tests as many times as you can until you feel your scores reflect your abilities. As AZcadet said, USMA will "superscore", or take your best individual score from each subject test area regardless of which time you took the test. Our son's admissions officer also indicated that while they look at grades, they also look at difficulty of courses taken. Because schools vary widely in their course rigor and grading scales, the standardized tests give a more complete picture of how candidates compare on a nation-wide basis, so yes, the test scores are also a very important parameter for the service academies in evaluating your file.
From our son's experience, I honestly believe a few C's are not going to keep you from getting a nomination or an offer of admission. That being said, some regions of the United States may be more academically competitive than others for nominations. It is hard to predict from year to year, even in a particular region or state. All cadet candidates are of course very competitive, but some may be stronger in areas of leadership than they are in academics, yet others may be stronger athletically than they are in leadership. Keep working in the other areas as well, and take positions of responsibility in activities that you love and enjoy.
When you begin the nomination process, your sources may ask for an essay. If appropriate to the subject, this is your opportunity to express your disappointment with your grades this particular year and how this inspired you to work harder the next time...to recognize your weakness and ask for help...etc.
Don't give up! If this is your dream, keep working toward the goal. Our son was in much the same position you are one year ago, and he is just a few weeks away from joining the Long Gray Line. It is not impossible. Just make sure you do this for YOU and not for anyone else