Help! I've lost all ual desire for my gorgeous and wonderful boyfriend! Why?

My bf and I have been together for 8 years. I'm 24 and he's 26. Within those 8 years we've taken a break from each other. I've been with other guys during that time. We've also spent a conciderable amount of time apart throughout the relationship because he's in the army. Right now we live together. Physically he's everything I could ever want. He's tall dark and handsome. We get along wonderfully. We're planning to marry in the near future. The only problem is that I've lost my ual desire for him. We used to have really great . He's a very pionate and adventurous lover and he does the things that I like but I've just stopped feeling the arousal. I normally have a pretty high drive and I know it's still there b/c I still masturbate. Getting rid of him isn't and option. He's the love of my life but I want our life back! What do you guys thing could be the problem?