Upset when he leaves..

So my boyfriend leaves on a weekly basis and is gone from Monday through Thursday due to his job being located on a base about 3 hours from here. Occasionally he has to take trips and is gone for weeks at a time. When I'm anticipating him to leave for one of these long times away I seem to shut down... like I get somewhat depressed almost like I'm mentally preparing myself to be alone.. and it really upsets my boyfriend that I do this. I don't want him feeling upset that the last thing he sees on my face is disappointment but I really can't help my actions or feelings at these moments. What in the world do I do?! Advice please...