Kinda sounds like my situation - living an hour away and not getting to see each other thing. Even though it's only an hour away, it still sucks. It seems like you're stressed and frustrated that you're trying to be positive (and stay positive), so when he's in his "low" points its frustrating because you just don't know what to do. I feel the same way.
I think the only way to work things out (at least a lil) is to talk about it and figure out a way you guys can work things out (preferably when he's not depressed); his job probably doesn't help with his emotions, since the job comes with a lot of stress and frustration, etc etc. and requires a lot of support (mainly emotionally)
But I think talking it out is the best solution to figure out where you guys are and to figure things out - like the distance thing. Communication is the key (so I've been told). It seems like you both need to support each other, but I think talking about it should establish what needs to be done and etc.

Sorry if i couldn't be much help, but I know exactly how it feels. Best of luck to you!