The morning after you break up with her will be the worst, most excruciating moment in the painful onslaught of emotional bruising soon to follow. She will deal with the pain by applying a powerful antiseptic known as denial. No matter how clear you were in detailing why you no longer love her (and hopefully, for your sake, you were explicitly clear), she will find a tiny, nearly invisible loophole in your words which she will worm herself into.

Did he say break-up? He definitely meant a short break. Did he mention another woman? She's clearly just a rebound. He'll realize how awesome I am and be groveling for another chance in a week, max. 

At this point in the game, she will also listen to misled friends who are ardently convincing her you would be a fool to let her get away, and that you'll return when you come to your senses, even if they're just lying to her to avoid having her meltdown over margaritas in Chilis. For the third time. That night. The margaritas themselves are also crucial to her coping at this stage.