Your guy, whom you are totally smitten with, somehow talks you into agreeing that it's OK for him to extend your relationship into a menage a trois including another, younger woman, that you don't really like very much. You're seething with jealousy, but, because of various abstract principles, you can't even admit it to yourself in so many words. So you write a novel based on real events, where you describe her as the empty-headed little bitch she is, and conclude by killing her, which you always wanted to do in real life.
Well... possibly there's a little more to it than that, but it's fun!