As for her screwing around with another guy...
Don't be a chump...
She did what she wanted to do...she didn't give a about you when she did it...she wanted to kiss this guy and she did it, was she worrying about her true love when she did it? No, she was thinking about possibly getting by ed this guy she is strongly attracted to...and it wasn't you buddy. Your little angel was seconds away from having her lips wrapped around another man's , and now she's got you feeling sorry for her...Sound like a recipe for happily ever after?
You still wanna be the nice guy?
She dissed you and ed you over...yet you are the schmuck feeling sorry for her and getting advice on a basketball message board...
Don't be a clown...this girl is turmoil and misery...she created this miserable feeling you have now...not you. And she'll suck you dry until you got nothing left and then dump you like you are garbage in your deepest misery(but she'll still want you to like her even then)
Do you want more of it? You like the feeling? You are going to get more of it.
There is no altruism in this sort of thing...if you think she's going to be eternally greatful for your benevolence in giving her a second chance...You are true chump of epic proportions...You're a nice guy...but do you want to be a nice guy so much you are willing to be a victim and chump to do it?
What she will conclude from you taking her back is that you are doormat and a sucker and she can pretty much get away with whatever she wants with you...You will never get respect doing that...you will get it even worse.