Most Helpful Girl
Having been That Whiny Girlfriend before, I have a few suggetions on how to save the relationship, if you choose to do so:
1) This may be totally irrelevant but just something to think about: Is she on birth control? When I was on the pill for about 6 months, all I always cried and got jealous over everything. No matter what my bf did I was unsure and suspicious. That could be the cause of her insecurity/mood swings.
2) If she accuses you of something (flirting with other girls, avoiding her, etc) she WANTS to be proven wrong. My boyfriend used to just stop answering when I got in his face over stupid stuff, which made me think that he really WAS flirting with that girl, and just didn't have a good excuse. Give her a good reason, otherwise she will blame herself and become more insecure and whiny.
3) It may be your personality. My bf is very quiet and introverted, so I don't get a lot of affection or "I love you"s. He just assumes that I know. Mistake. Don't assume she knows you like her or think she's pretty. Many girls know that puppy love wears off, and if you don't give her a compliment here and there, kiss her goodbye, or ask about her day every so often, she's going to be scared you're tired of her. Even if you think it's redundant and unecessary, girls like to be reminded that you care.
4) Surprise her. Make her a fancy dinner when she gets home. Have some surprise sex in a secluded area. Take her on a super romantic date (dinner is fine, but something fun like amusement parks or a concert of her favorite band will earn you major points). Plan it by yourself. The fact that you put the effort into planning a big date and the anticipation when you tell her you have a surprise for her, will surely give her some faith in you. Do something to show you listen and know what she enjoy.