Leaving for the military and my girlfriend won't stop screwing with my head. What should I do?

First of all I feel obligated to say that you shouldn't believe that the military is your only option. Never think that and just do what makes you happy, now what you think you have to. Second, I would understand her being upset about you leaving but honestly the piece about her killing herself seems a little too much and a little too clingy. It seems like she has some other relationship issues. If you are wanting to help her out with those in a healthy way other than just being there, stay with her. If not, let her go. Do not let your decision as to whether join the military or not depend on her. Do what makes you happy and if she truly loves you she will be supportive.