Military girlfriend help!?

I leave for the air force in 4 days and my girlfriend is reluctant to get intimate with me. i get the vibe she thinks im going to ditch her and move on which is not the case. we've only been going out for about 4 weeks and i can understand attachment issues when i leave and she goes off to college. and i know i wont get to see her alot and likewise but do i just switch our relationship to "just friends" cause its really hard were really close but she puts these guards up like she's bracing for impact.HELP!