This is just my opinion: in the best interest of your marriage, please let the girl fly away. Do you and your wife have any kids? It sounds like you're treating this girl like your own daughter but unfortunately she doesn't see you like a dad. She was in her teens (18) when you "adopted" her. You bought her a new car, let her live in your house, help paid her tuition - everything that a parent does. It is possible you have come to dote on her like your own child. She probably mistook your affection the wrong way and came onto you. Your marriage is threatened and you admit it's hard to resist her. It is probably best to let her go, out of the best interests of everyone involved. She is already an adult so you are not responsible for her anymore. Let her live her life.