Summarize this article:

If your girlfriend seems depressed, recognize it as a problem. Don’t brush it off as something she should just “get over.” If she’s already getting treatment, don’t think that the problem is solved—continue to offer support. This is especially the case for chronic depression. Those suffering from this form are prone to relapses, even if they appear to have been well for a while. Caring for someone with depression can be hard on you, too. You can’t fully support your girlfriend if you aren’t at your best, so make sure to practice good self-care.  Make sure that you’re eating well, getting enough sleep, and generally taking care of yourself. Seek help yourself if you begin to experience depression or other health issues. Don’t engage in risky behavior, like drug or alcohol abuse, to help your girlfriend feel better. Keep living your own life. See your friends, go to work or school, and have fun. People develop depression for a variety of complex reasons. It’s never as simple as saying that one thing or another “caused” someone’s depression. Don't fall into the trap of feeling like something you may have done caused your girlfriend’s depression. Instead, focus on her treatment and recovery.

Summary:
Take her depression seriously. Take care of yourself as well. Don't take it personally.