RSG,
There is no way to tell what is the right thing to do. It is a very hard time for you, but neither of you are committed and technically what your GF did was really not a broken vow. I know it hurt but the reality this is why people date and yes date around.
To give you an idea how old I am I went to an all male college (none of those exist anymore). Virtually everyone there got what was call "Dear John" letters from girls who they were going with before coming to college or had met over summer breaks. Indeed it was so common that the school newpaper used to publish them along with a "critical" analysis as to originality, "thoughtfulness", cruelity, and just plain silliness. The one judged to be the best led to the guy getting some sort of prize and the girl being sent a dozen roses if I recall correctly.
Now why am I telling you this shaggy dog story? While times have changed people haven't and what you are going through is pretty normal. In fact, you should and I really mean this be happy it has happened. Why? Well, in my experience many romances that start in college and lead to marriage that fail. Why? Because frequently all the couples had in common was studying, tests, and the social life that the college/univeristy provided. When out in the world facing the challenges of making a living, having children, handling money, many of these couples found they had little in common.
My advice, do your best to heal from this. Finish your school, and get out on your own ( I suspect you have done some of this having been in the military) and see if life brings you back together. I suspect but you didn't say, that your GF was experiencing her first taste of freedom (ie: her own money, no studying at night, living on her own, etc). Her behavior is not surprising in the least.
I understand that you are deeply hurt, but please recall that although you may think she is the girl of your dreams and you are ready to settle down, she may not feel the same or be ready to settle down.
So calm down and see what happens, but don't neglect your studies, don't neglect YOUR future. You have a long one ahead of you.
God Bless,
JL