Long story short, I broke it off because she wasn't handling things well at all. I'm currently finishing college, and she opted to stay at home with her family until after I graduated. After about 8 months she began getting entirely too clingy, needing to talk to me daily on the phone or else she'd freak out. I sat down one night and tried to talk things out with her, and it became clear to me that there was no way she would be able to handle my life in the service if we were to marry. If she couldn't handle a year away from me while I'm at school and only not being able to talk to me for a day or two at a time if I was busy, how would she handle things like deployments or a war. It just wasn't healthy for either of us, so I called it off.
So technically I guess you could say the Marine Corps had it's hand in ending my engagement, albeit indirectly.