<human>: Can you condense the text into a few key points?

Background; we have been seeing each for a year and are living together now.

My girlfriend goes through bouts of what I would assume is some form of depression, but of course I'm not a doctor. During these times (which can range from a week to a month) she closes herself off from me and sometimes is emotionally distant. When she does talk she says things like "I just don't feel anything right now" or "I feel worthless". Whenever I try to talk to her about it she will either insist nothing is the matter, or just tell me she doesn't know what to say. Lately I've been encouraging her to just talk to me about it, even if we can't figure it out, even if we don't end up anywhere because I assume having it out in the open more often will make it easier for her deal with it. But I am also not a psychologist. She has seen a few in her life but we don't have that kind of money at the moment.

I end up feeling distant from her because she won't let me in, and she ends up feeling bad because she believes her attitude is making me unhappy. I realize it is not my job to fix her, but it does effect me and I don't know what I should be doing. For obvious reasons it is stressful for me to come home to someone who is feeling negative all the time, and effects my mood as well. I just feel ill equipped to deal with these sorts of problems and I am afraid I might say something or do something wrong.
<bot>: Girlfriend is emotionally distant, and I don't know how I should deal with it, or what I should be telling her to help her open up more.