Okay, so first of all, you can't control who you love and why is she the wrong girl. Maybe she is the wrong girl for you. You shouldn't hang out with any one out of sympathy, that is what is wrong. Not the girl, not you, but the sympathy.
If you feel sorry for some one, you pray for them. This is how it is. So, you weren't really attracted to her but some how you were leading her on but you didn't mean to do all that. You were just trying to be her friend. I mean, I can understand being some one's friend, but don't be that friend who feels sorry for them. Trust me, people don't really like that.
They know when you feel sorry for them. They know. Your falling in love with her, and I have no idea why. Maybe it's because opposites attract that's why. So, you look down on her past, really why. It's not your life nor your past. That sounds more like you were trying to make her your girlfriend, then just trying to be nice to her. Just saying. Maybe that's not really what you were trying to do, but from my point of view that's how it seems to be.
How did she run away and still be clingy at the same time. See, that just sounds so confusing really. So, now your in love with her and you don't know what to do. But, you were only your friend because you felt sorry for her. So, you start "dating" her and now your in love with her.
Well, I'm not too sure what I should say to this, but good luck to ya! : )