I left her, I lost my love for her and found true love. Why can't I get rid of the pain when I think of her?

I lived with a girl for 9 years. I never truly loved her but I stayed with her even when everyone around me didn't understand. I left her and found my true love. I'm OK for the most part, but every once in a while I have great sadness when I think of her. My true love tells me she didn't care about you and treated you like dirt, why are you concerned. I've had friends and family tell me the same thing. But still I have moments in time I am very sad when thinking of her. What is wrong with me, and how can I forget. It's been almost a year since then. My true love and I have been together for 6 months and we are doing EXTREMELY well. Help me to forget.