breaking up with my fiancé...

Mariah
My fiancé and I have been together over 6 years, engaged for about 14 months now, too. We were best friends before our relationship, and I want us to still be friends after.
I love him to death, but over the past two years we've been growing apart, and I'm confident now that "spouse" isn't the right role for him in my life. 
Has anyone ever been here? Or maintained a positive relationship after a breakup?