Speaking from the other side, as a military wife, it is VERY common to have a major fight before your loved one leaves, for whatever reason-ex, deployment, course. It is easier to be mad at someone when they are leaving even though know that it is your job, it doesn't always feel good knowing that we are not your first priority. Even though my brain knows why, my heart still hurts. :-[
So, being a girl and emotional, as we can be sometimes, we will just make your life miserable >
It's dumb, but it just is. After almost 14 years of marriage my husband and I joke about our big fight before he leaves and we try to avoid it, but it ususally happens :'( But, we make it through and all is good in the end.
If it's meant to be, your girlfriend will be there when you get back. Or maybe your girlfriend and the military are not a good mix or maybe they are, it is all new to her too. Right now you just need to be honest with her, let her know how you feel, but if the military is what you really want then you need to go for it! Good luck