Ask Roe: I went on a few dates with him and then he got back with his ex-girlfriend

Dear Roe,    
I have an obsession with a man I know. Three years ago, I went on a few dates with him and then he got back with his ex-girlfriend. He was in a relationship for three years and he recently broke up with her. When he got into a relationship, I set myself boundaries, told myself I deserved someone who was 100 per cent interested in me and tried to train myself not to think about him, see him, etc.