If you like this girl let it go MAN! Who gives a fuck who she was with, that was then! The past is nothing but an elusive flicker of non-existence. You have the prize, so celebrate. Do not punish her for this, what if she did the same to you for your past? It is not right. True confidence in a man will always be more valuable then ANY OTHER ASSET! If a woman sees you have insecurities about the past she will definitely see you as unconfident and her respect will wane.