How can I handle him leaving for the army?

My boyfriend leaves for basic in less than three weeks. It's tearing me up inside. I'll miss him so much...he says he'll understand if I can't wait for him. But it's not about me wanting to leave at all. Why would I leave him? I'm upset because I want him to stay. What sense would that make? I wish there were a way I could express the distress I feel without making him feel like it's too much for me. There's no question about my staying faithful. But that doesn't make it any easier. How should I cope? What should I do? Is the anticipation worse than the actual event in this case?