I've never been diagnosed with depression, but I've felt pretty bad for long periods of time in the past.

I found that getting into a daily routine really helps. You can feel as if you are in control and you know when you have things to look forward to. It helped me to just calm down and get a grip basically, then I can return to life at my own pace.

Obviously you can't really tell your girlfriend this if you don't want her to know people believe she has depression, but maybe you could regularise when you see her or something along those lines?