It's not just unfair to yourself to stay hung up on an ex; it's also hugely unfair to other people who come into your life, people who might be looking for just you. You know your ex has made a definitive choice to go, but you don't know who might be waiting to make the choice to come in or stay. Let the door be closed. Give them the uncompromising exit they asked for. "If they're dumb enough to walk away, be smart enough to let them go."