If you are a medical student, remember this: 90% of the stuff that you learn in the first two years of medical school, you will eventually forget and never use again in clinical practice (this is assuming, of course, that you are going through a 'traditional' program). The other 10% will be drilled and pounded into your brain ad nauseum until you learn it. Your clinical rotations during your third and fourth years (and your residency and thereafter) will provided that "pounding".
When I was a medical student (a long time ago), I use to get stressed out because I couldn't retain all the information that I learned in my basic science courses. But as I moved from medical student to resident to attending, I slowly realized that almost everything that I needed to know was reinforced over and over and over again—and I didn't need to learn and retain everything with the first try. So take heart, take a deep breath, and do your best to enjoy learning about the wonders of the human body.