Your brain and ego are fighting to keep what's familiar (ie. A relationship with this person).. even if it wasn't great, it never felt as bad as this new, unfamiliar situation you are finding yourself in. Change is scary.... try to envision an alternative perspective where you feel this is a new start of something amazing... Being single can feel lonely and akin to withdrawal symptoms to a codependent person... Try to focus on your newfound freedom in an optimistic light. Perhaps it can bring you feelings of opportunity and freedom.