Are you sure you want to trash the relationship? It didn't sound too bad at the beginning of your post. She wants to KNOW that you are really sorry and that you really FEEL that that choosing the other girl was a mistake. Be honest with her and tell her why you did it (because you weren't thinking) and tell her that you still have strong feelings for her. If you are sincere, she MIGHT understand, but don't give up, she wants to see how sorry you really are.