The short answer is WE CAN'T. The best way to fix someone else in a relationship is to focus on working on yourself and how you feel. The journey of relationships is the same journey as the creation of who you really are. When you have lost your zeal for the relationship you are in, it is time to reflect on why you chose to be in this relationship at the beginning. Was it on purpose or did you just "fall " into it. Taking responsibility for our own feelings is the best way to be true to yourself and that is the best place to know whether you are in the "right" relationship. Any relationship can work when both people make it work.