First of all, cut a hole in your pants and make sure its directly inline with your cornhole. Call her up, and say you have a really important question to ask her, and see if you can come over to her place. When your inside, have her sit down on the couch and kneel down in front of her. Grab a hold of her hand and close your eyes. At this point she'll be convinced that your about to propose to her....but shes wrong. Keep your eyes closed and act like your really trying to find the words, but what your really doing is taking a big shit on her floor. When your finished making your poops, stand up, kiss her softly on the lips and run off crying.
Once she realizes what you did on her floor, you wont even have to worry about breaking up with her.