It's when you don't care anymore. You feel almost neutral. Whey you find yourself focusing on your ex's feelings instead of your own, force yourself to focus on your own feelings. It's natural to think about what your ex might be doing or thinking. Try to think more about yourself and your own needs.
People do different things - take baths, drink tea, watch favorite movies, play with a pet.
Do whatever soothes you. Do things that you have put off for a long time. Don't over indulge in drugs or alcohol. You might do something you'll regret and end up feelings worse. While feeling down you may be tempted.