I'm not a party-goer type. I think I've been to three in my entire life. I am also one of those rare breeds who didn't have a drink of alcohol until I turned 21. And on my birthday, I consumed two beers, WOOOO!
I relax and have fun pretty much every day of the week, though. I've never felt stressed in UG (even while studying for the MCAT during fall semester of junior year). Keeping everything in perspective and knowing what is really worth worrying over is key.
For instance, a friend/relative passed away? Yeah, that's stressful and extremely sad. A C- on a test? Don't start crying and thinking your life is over. Move on and do better.
Most importantly, realize that you get one shot at life. Be sure to do the things you love. Sure, becoming a physician is a dream and be sure it happens, but don't push what you want to do to the side all of the time.