AMCAS will use all your grades when computing your GPA anyway–both the original and the retaken grade. Allopathic med schools (MD) don’t allow grade replacement.
The learning/academic support center at your college probably offers a study skills seminar/class to help students learn how to adjust to college workloads. Look into that.
Your college also has a tutoring center that offer free tutoring for all basic and intermediate level classes–like gen chem and intro bio. It’s a free service–use it!