That's really sad, and probably the underlying fear running through this whole thread. Conventional wisdom says you should have met somebody equally engaging as her by now... I didn't think situations like yours were very common at all.
You are right. It shouldn't be very common. I guess I was unlucky (or maybe lucky to have had that kind of relationship at one point in my life?). It's not that I was unable to love anyone else, it's just that that person was so perfect for me. Do understand, though, I'm not stuck in the past nor am I willing to sacrifice my life for that memory. We are still friends, she is married, I'm living with my girlfriend whom I love. All is well. But I'd be lying if I said I never think about how things would have been if we stayed together.