A: I give you props for trying to save this relationship but it really seems done. I'm really sorry to say that. But if she loved you enough she would forgive you.
A: I'm sad to say but there's no nice way or light way to break it down to her, if it's hard to it in person then do it over phone. ( NOT EMAIL. ) You probably have some feelings for her that's why it must be hard for you to break up with her.
A: Well she is dating so you don't want to come between that relationship. But maybe in some time you might now even like her so instead of going at it, leave it alone for a bit.