you are going to feel really very guilty but you have to lose your friend. and come straight and honest with her. tell her why you cant be with her and that your wife knows about it. tell her it was all a mistake and be sorry for it. seriously, this is going to be very hard on her on you but the position you are in you have to choose betweene either of the women, you cannot sail in two boats, you are going to feel guilty and resentful but remember always what you did was right for you, right for her right for your wife right for your marriage and your family.