> It does feel as if this regresses for long-term partners. The volume of shared experience must blur the boundaries of individual experience, or something.

This is so true, and painfully so. I was fortunate to have an epiphany a few years ago when I realized I was failing regularly to see my wife as a separate individual and part of the whole of "us."

More accurately, there were times when I probably viewed her merely and extension of myself. It honestly changed the way I view the world, and I somehow managed to extrapolate that understanding to life itself. It was a glorious dose of ego death, and one I sorely needed.