How do I tell my family I am going to deploy rather than stay in college?

I'm an infantryman in a National Guard unit that is being activated for deployment to Afghanistan in a few months. I've done ROTC for two years now, and my family expects me to stick with it and commission, but it just doesn't feel right to me. If I stayed, I would have a waiver from going due to ROTC, but I would hate myself for doing that if I stayed back. How do I tell them that instead of staying in ROTC and becoming an officer I'm going to the sandbox to do my duty and probably never become an officer like they want me to?