Tell her how you really feel, tell her how much she means to you and that you would never want to be with anyone else or see her with anyone else, just sugarcoat everything you say, and when she is upset, say you are sorry, even if its not your fault and you did absolutely nothing, let me know how it works out.
If she wants to break up with you theres really nothing you can do, but you could try being nice to her, try not to shout, girls hate it when a boy shouts at them, Oh and if you want to be romantic, buy her a box of chocolates and some flowers and say your sorry. Good Luck Hope This Helps!!! =D
Tell her you were wrong about yesterday, and she means too much to you to just let her go so easy. Fight for her. Make her feel loved. Hell, buy her flowers and chocolates. If the fight was bad enough that she wants to break up, you've got some apologizing to do if you want to be with her.
Sai: you're retarded... Stop trying to act like a thug. You sound like you're about 15.. Don't give anymore advice about girls, you know nothing.
Stillfly: apologize to her. It may be that time of the month, or maybe she's stressed or thinks she deserves better (she might be right). But if you love her, admit you were wrong, promise you'll try to do better by her, then DO BETTER by her! If you love a girl, you do what it takes. And don't listen to these moronic wannabe thugs anymore.