Or not.
Bottom line: something's got to give. Your girlfriend needs to know that you are not the only source of her happiness. And you need to know that, if you want to keep someone like her (and perhaps she's not the right person for you), then you need to bend a bit as well.
It's a nice idea to want to do something nice for her while you're out (e. g. send cards or whatever), but she needs her own inner resources to keep herself together while you're busy with other things. What does she honestly think happens when someone's spouse goes to work in the oil fields for six months, or off to war? Pining your life away because you've got nothing going on while you're man is away smacks of a clinginess that would probably drive most people nuts after a short time -- both the clinger and the, er, clingee.
I was brought up to have my own life, my own job, my own thoughts and my own ideas and means of keeping myself happy. If my husband were away for six months (or whatever the period of time is), I would miss him but I would still go to work, see my friends, plant the garden, cook the meals, etc. etc. etc. In short, I would go on. Your girlfriend needs to do the same.