I have a friend of mine whose relationship looks to me like the emotional rollercoaster from hell. When it's good, it's very very good; but when it's bad, it's horrid. From what I can tell, his girlfriend has a very high analytical intelligence but lacks emotional intelligence. Whenever she's upset, she treats him like crap and he has a tendency to let her walk all over him. He really likes her so he swings between being happy as Larry and anxious as all hell.
Recently he's started to man-up and tell her that it's not OK for her to treat him like trash. When he does this, she settles down and starts to take responsibility for her own feelings instead of just dumping on him. Then later on she blows up again.
The irony is that the closer he comes to calling it off, the more she's on her best behaviour and the kinder she is to him. I fear that once she's got a ring on her finger, she'll relax back into her old ways and he'll have a lifetime of mistreatment to live with, too guilty to divorce her while placating himself with the belief that he doesn't deserve any better. Bit like my Dad really. So to help out, I've made this video on How to Break Up With Your Girlfriend: