Nearly every premedical student has struggled in a course from time to time. The best strategy when that occurs is to utilize the academic services that Swarthmore College offers, such as tutors, biology and chemistry clinics, Writing Associates, Student Academic Mentors, and study skills workshops. Take advantage of faculty office hours, or time before and after class, and ask your professor to explain something you don't understand.
An important skill for being a doctor is knowing when and how to ask for help. Don't let a small problem grow bigger because you hope it will go away on its own.