Same situation, my boy is away at uni and like u he finds it a lot easier to cope with than i do. we see each other about once every 2 weeks.
My advice to u is to make sure u call and say goodnite, little unexpected things aswell like actually writting a letter and posting it (not an email) between visits. just be extra sensitive to the fact that she's missin u and let her no that your missing her aswell.. thats important!
My advice to your girlfriend is to try and focus her time on something else. At the moment i'm pretty much doing 6 day weeks at work, and i joined the gym. So not am i only so knackered that even if i was with my boy i'd be too tired to notice lol but by spending loads of time in the gym i'll look fantastic when i next see him ha ha ha.
hope this helps..and if ur too scared to tell her that she should find something else to worry about, just show her this reply... coming from another girl might be easier lol.