Happy Birthday Jil. You're right, it is extremely difficult to stay sober in college. It often seems like the only fun thing to do. Furthermore, I think that most people don't realize how much they are drinking, because it seems that all the people around them are drinking just as much as they are (and this might be true in some cases).
About a week after my 21st birthday, I walked out of my job as a cashier at Toys 'R Us in the middle of my shift. I just could not handle being in there. I called my university's health center and set up an appointment in the mental health department. I saw a psychiatrist 4 or 5 times.
I remember taking some test to measure the volume/frequency of my alcohol use. I was of course off the charts, but I refused to believe I was an alcoholic, because all of my friends drank as much as me. I also remember my psychiatrist recommending that I try quitting drinking, to which I agreed, and I wound up getting wasted just a few hours later. Funny thing is, I didn't feel guilty at all. It seemed perfectly fine to me, despite all the bridges I had burned with my behavior.
So, pay close attention to the way you REALLY feel about yourself and your behavior. Think about the big picture, not just what your friends are doing. It is very difficult to do, but it sounds like you already have a pretty good idea what is best for you. Congrats.