This obsession isn't healthy. This situation isn't good for you. Let it go. Let the resentment, the revenge go. Move on with your life. Treasure the fond memories and release the pain she caused too. This was an experience and chalk it up to that and learn from it. Continue Al-Anon meetings I would say as you seem to have some codependcy issues.
Bravo to you for leaving. If you bump into her or him....a small smile and a hello is sufficient and just remember that you are choosing a HEALTHY life.