Of all the books and articles about love and relationships, the best description of what love is and what love is not still comes from the Christian Bible 1 Corinthians 13: 4-7 (in my humble opinion. This posting comes from the Good News Translation (United States Version).
Love is patient and kind; it is not jealous or conceited or proud; love is not ill-mannered or selfish or irritable; love does not keep a record of wrongs; love is not happy with evil, but is happy with the truth. Love never gives up; and its faith, hope, and patience never fails.
Look back on past failed relationships and compare the relationship with this short list of traits of love. You may discover why the relationship failed.