Girl friend crazy or me?

I have been dating a girl for over a year and I am starting to think that she is emotionally needy. She is constantly getting upset she sees things in a very negative light and always talks about bumps we have experienced in our relationship some of which happened over six months ago, I have always sincerely apologized for when I have messed up, but at times I feel as if im apologizing for things I don't really know I did or that im not responsible for. I have never cheated on this girl or even tried to make her jealous I try to be really nice and let her know im there for her and I really care. but the constant nagging and getting upset are really starting to take their tole on me. I really love this girl and i need to know if there is anything i can do, she gets upset and tells me she's angry at me about 2 to 3 times a week (she also cries a lot) and she will get really mad but then be really sweet . When she's not mad or upset she is a really sweet person, which im scared might be blinding me to the reality of my situation. Am i over reacting to this or is it a cause for concern if you have advice id love to hear it, this relationship means a lot to me but so does my sanity.