So you are joining the Army in hopes of spending time with her and seeing her overseas in combat? I admire your devotion to your girlfriend, but this is a bit much. Do you even want to be in the Army if she wasn't? It's an outrageous reason to put your own life at risk, your own hopes and dreams on hold, just to MAYBE....POSSIBLY...see her over there?
I understand you love this girl, but there is zero rationality to this. Not only is she eight years younger and at a completely different stage in her life as you are, she chose to undergo this path of military service and these are some of the consequences of these actions. There are so many reasons why relationships just don't work out, and this may be one of these times. If you do pursue this, you are signing yourself up for years of service, and may not get to see her. Or you may overlap and have to serve while she is home. It's not worth it man, as I'm guessing this is the only reason you are joining.
If you know what you want to do with your life, you should probably find a way to do it without her. It hurts and it's painful to let something go, and she may be completely scared shitless and feel lost without you. But this is what she wanted to do, and if it doesn't go with what you are doing with your life, don't put everything on hold. You aren't getting any younger, and if you go through all of this and it still doesn't work out later on in life, you are going to harbor such resentment for her and feel like you lost so much time. You and her working out in life aren't guarenteed, but your future is completely in your hands. Don't sign up for more than you are bargaining for.
She may feel like the only one in the world for you, but she isn't. I'm not trying to be pessimistic and if you really want to do this, there is nobody stopping you. Really think this through and weigh it out with complete rationality.
Waking up next to a beautiful girl,
Step outside and say hello to my beautiful world.