To me this song means- a woman and a man are soul mates and having an affair but he's married and won't leave his wife even though they are meant to be together. Then she tells him to stay with his wife even since him always gong back to her and it gets too hard. I think the moral is be careful who you fall in love with and love is hard and painful. Although they are not together and are not even having an affair it reminds me of Elliot and Olivia on Law and Order: Special Victims Unit because although they have never admitted their love and Elliot is married that is what their relationship is like- for me anyway.