The speaker has fallen for someone who is still hung up on an ex. Standing at the mirror, they greet themselves with bitter sarcasm: "congratulations," they've been promoted from side-chick to rebound, which sounds like a step up but isn't. The person they love can only think about someone else. Even so, the speaker doesn't turn nasty. They insist that the other person deserves a fight, even a war, and that all they really want is for them to be happy, even if that means not being the cause.
What stands out is the structure. Halfway through, the poem repeats itself almost word for word, but "side-chick" becomes "playboy," "he" becomes "she," and "him" becomes "her." The swap suggests this heartache isn't tied to one gender or one kind of person; it can happen to anyone. The repetition also feels like a loop the speaker can't escape. The mood is sad, self-doubting, and tender, shaped by the painful fear of being "nothing new."