how can I be a friend to my ex-lover?

I need some adviced on how to be just a friend to an ex-lover without treating her as if we are still in a relationship. I Truly just want to be a friend to her and nothing more. You see in my heart I was force to this. I'm very much still in love with her but she's loving someone else and nothing hurts more then to watch the one you love love some one else. But she says that she's still in love with me too but actions are differently then when my heart felt it to be so. I dont doubt that she don't love me but still in love with me no I don't beleive so. I just dont want to be in love all by myself because it's no fun. My mind wants to let go so she can continue to be happy without me causing conflit where it shouldn't be any. If she has moved on then why can't my heart do the same. Maybe it's the years that we had together. I guess I should mention that we do talk ona daily basis as friends but always arguring about her new lover. I don't want to do that anymore. How can I be a friend