How can I reduce stress as school begins?

All the people in my major seem to breeze through homework, tests, and absorb every lecture like they've heard it a million times. This leads me to believe that I might have chosen the wrong major, even though it is what I really want to do. I'm already a senior, so changing majors now is out of the question for me, since I've already made all the arrangements for graduation. I know I will graduate with a very low GPA... not even high enough to get into graduate school in my major. The coursework that I'm headed towards in my last two semesters seems like total rocket science to me and asking for help, even from close friends in my major, is very embarrassing. I normally wouldn't mind asking my friends for help, but they have very belittling things to say about other people who don't understand the concepts. Plus, everyone that tutors for my major is also in my classes, so asking for help there is also impossible. I feel that if I don't get help and start understanding things essential to my major, I will completely abandon the idea of going to graduate school, or even trying to get a job related to my major. I don't want four years of schooling to go to waste... what should I do?

I've always been something of a hypochondriac, although my friends and family know nothing about it. As a child, I would regularly convince myself that I had appendicitis. As a teenager, I worried about being pregnant despite never having had vaginal intercourse. As a college student, I worked in a hospice for late-stage AIDS patients. Although I never had contact with blood or semen, I convinced myself that I may have contracted HIV somehow by doing their laundry with some kind of minor cuts on my hands I never noticed, or because a patient drooled on my hands. I've been tested at least three times since then, but I still haven't gotten over it and am nervously preoccupied with the idea that I have infected the whole two people that I've ever slept with. At the moment, I'm also convinced I have cancer. I *know* I'm completely wrong about all of this... I know, I know. And get this: I'm in medical school now. It's only going to get worse. Help me!