What is Fergie's song "big girls don't cry (personal)" really about?

I mean, she says "I hope you know this has nothing to do with you," but that doesn't seem right. Doesn't it always have something to do with the other person when you decide to leave? Sure, it might be that they are perfect but you just aren't happy for whatever the reason, but then are they really perfect? Anyway, thoughts on the song (as in, the meaning behind the song) are appreciated!