I think my gf is depressed. Please help

Well from the start. Her parents split and her dad really did not have a part in her life. Her mom kicked her out of the house at 13. She was pregnant at 16 and had a beautiful daughter. She is a hard working girl and smart. She is 22 and has her own appartment. I come from a very loving home with great loving parents. Totally opposite to her. We met and started dating a girl in the summer. when we met first two months were amazing. Then all of a sudden things went bad. She texted me saying she did not want to see me again and we were over. We had a little argument the night before. I was devestated. I sent here flowers and a had written letter. She accepted it and thanked me. What I found out was that she had just quit her job. She stopped talking to her best friend. She just shut herself off. She said she needed space. So I would just send her flowers a few times a month with an encouraging letter. She would text me now and then to update me. Then mid december she texted me and said she missed me and wanted to see me. I learned she just got a new seasonal job. Her spirits were up. We seen each other several times and things were good. Then when the seasonal job was over she just shut herself off again. She would send me a text saying she needs more space and hopes things will get better soon. So I did not bother texting to give her space. She then sent an angary text saying that she figured I gave up on her. I was just trying to give her the space she wanted. Then last week we got together and she said she wanted to take our relationship to the next level. I found out that her older sister of 32 just got pregnant from a guy she met a month ago. She went with her sister to meet him. This went bad and they had a big argument with him. After that her attitude changed again. She told me she did not want a relationship anymore. Things are so up and down I get so confused. I remeber saying that some of her siblings have depression issues. I then started reading forums on here were other people experiencing the same things. I figure now I have to give her space again. She is the only girl I have truely fell in love with and I want to be there for her. Please help with advice.

Thank you for your advice so far. I have figured she needed therapy. I just so scared to tell her this. She is a smart and indepantant girl. So if I tell her this im afraid she will take it wrong and be upset with me. How do I tell her this in a gental way?
Deep down I kinda know she is reaching out for help. Right now she reads and writes alot about philosophy. Such as philosophy on love. She talks about it non stop. She tells be how she loves that I always try to make her happy and that she knows that she would be happy being with me. She says right now she has to create her own path. I also remember her saying that she does not remember alot of her childhood like she has blocked it out. My heart is broken for her. With trying to find a job, being a young mother on her own, and family issues, I can not emagine the stress and pain she goes through.