Replying to Dealing with depression in others

Hi NS, so I'm in a bit of a dilemma. My girlfriend and I have been dating for a little over a year, and we have our ups and downs. I really love her, so this is what makes it hard for me but she is dealing with depression or something, I don't know. She breaks down and cries about once a week. She says everybody gets sad and that its normal but I don't think it is. It's normal to be sad every once in a while but not weekly. And she gets sad 0-100 real quick. I beg her to come to therapy sessions with me but she won't do it. I try to help her get better but I just can't seem to do it. And having to cope with her being sad a lot has taken its toll on me, I feel like I'm turning into a caretaker and I have to fix all things for her. I imagine it as this one game on the Xbox Kinect where you are in a glass box under water and all these holes get poked and you have to cover all the holes our you'll drown in the water. I just get overwhelmed sometimes and get major anxiety. And I feel as though when I'm sad and breaking down because of something I have to deal with it myself so I can tend her wounds because she will start crying and getting sad. I don't want to leave her because when she is happy it's amazing and she is such a kind, genuine girl. She's not a total bitch and she's one of the few girls that aren't crazy. She's just sad, and it breaks my heart. NS isn't really the ideal place to look for advice like this, but I guess I just need to vent and look for some answers.

Summary: My girlfriend is dealing with sadness or something, and I don't know what to do about it. Plz help.

Props if you got through the whole thing.