Girlfriend been gone for a while now...wandering eye?

So my beautiful girlfriend of 4 years joined the navy 2 months ago and has been at boot camp with very little means of contact with me. I love her to death but there's been such a communication error its been putting me off emotionally. I'm 22 and my girlfriend is 24. We've been together since I was in high school. But there's this girl at my school who is 18 and we are becoming friends and I can't stop thinking about her. She is soo gorgeous, but a bit younger than me. I really really can't stop thinking about her either, its weird...but I love me girlfriend. What the heck is wrong with me and what do I do?? ;(