I know that is a burning question with you: "How could she have moved on so fast?"
The truth of the matter is she probably hasn't. You've heard of rebounds right?
You trusted her, put alot of faith into her words, but just because she says "I love you" doesn't necessarily mean that she loves you the way you feel love. Love is our own interpretation, and some have a very, very, VERY skewed definition for that. I would be the first to admit that I never really understood it and it has taken some dating experience to help me better develop the concept of love. And every serious relationship has helped me develop a more advanced relationship than the last.
She may have handled it immaturely, may have been dishonest a bit about how she was feeling, but you can't blame her for wanting a relationship with somebody that is going to be there. Most people in their right mind wouldn't want to be with somebody they can't be with, only see a couple times a year, if that. Sure we have skype and phone calls and all that, and I'm sure you would love to have somebody to share special moments with, but it's just not worth it to her.
The question shouldn't be so much "How can she move on so fast?" You should be more concerned with what you got in front of you. Years of service, combat, all those fun things. Good luck, soldier.
Waking up next to a beautiful girl,
Step outside and say hello to my beautiful world.