well, first you need to make sure it was a relationship in the first place. after expressing your deepest secrets and feelings to her, you need to make sure that she knows how you'd climb mountains for her, and that you love her more than you ever thought you possibly could. you need to make sure she knows that shes the only reason you were ever even considering staying in your god-forsaken town, and that shes breathed life into your otherwise stale and empty lungs. then, after a completely meaningless and average fight, you need to make sure she dumps you first, as you bleed yourself dry for her, and explain how no matter what happens you'll always hold her close and stay by her side. soon after you'll realize that she never really fucking cared in the first place, and after all that had happened between you, she seemed to carry on as if it was all meaningless and was just a lie. you'll begin to burn any bridge to your heart, build up brick walls, and ice over, just to make sure you never let anyone in again. and then when people ask why you've become such a womanizing asshole, you can simply explain that it never would have happened if a girl didnt play with your heart first.
but hey, why listen to me? im just an average guy who has really loved and lost. take the advice of every 13 year old on this site, you'll be fine.