Welcome inchiki,

Your girlfriend is lucky to have such a caring and involved partner! Thinking about your situation a bit more, it isn't that unlike that of mine and my SO, so I will give you a couple of thoughts. I apologize in advance if they seem a bit silly

I am the depressed partner, and my SO tends to be quiet, so he isn't always able to distract me - but we have developed our own "silly/fun" stuff to talk about, and we sometimes play games like trivial pursuit which can be good for distraction. Another possible idea is to look at books on depression together (we haven't done that because he has a very stressful job and wants to relax in the time off that he gets) ... The most important thing is to be there for your girlfriend, and it seems that you are doing just that!

Take care,
Fuzzy