CS - I'm new here and totally relate to how you are feeling. My fiancée was my best friend too. I'm stuck in the obsessive part and analyzing WHY this happened.
It's so hard to let go. It feels so foreign. I watched my fiancée pack her things and walk out the door tonight. The only way to describe it is like she reached into my chest, ripped my heart out and then walked out, got in her car, and drove off.