Of course breaking up hurt because real emotions are involved unless of course you never had them to begin with than it doesn't matter. Your girlfriend's heart may be broken but what happened between the two of you is a blessing in disguise. Sure there are plenty of girls but the ones who truly loves you is rare, keep that in mind. if you know you don't love her, you would do her (and yourself) a big favor by leaving. Don't waste anymore of her time, tears, etc.. She's hurting now but one day she'll thank you for today.