I hate the rollercoaster. And I know I'm not alone. I'm not sure what saddens/disappoints me more ... the physical part of the affair or the other aspects of the secret life (places gone, experiences shared, etc). Months prior to D-Day I worried aloud to my W that I feared being left behind and now I see that, in some respects, I had been.

Am I alone with this thought?

Nope! It's a feeling of being alone...when you're with someone you love. Hard to bear.

Whip --
You wrote:
Months prior to D-Day I worried aloud to my W that I feared being left behind and now I see that, in some respects, I had been.

I can seriously relate to that feeling, and I think that's what troubles me about your post. For YEARS I said I worried I would be left behind, that she would have an affair, that she would divorce me or leave me.
I've come to realize two things: One, I was protecting myself from what I thought was the worst thing that could happen. By predicting the behavior, I was preparing for that day well in advance, and kind of giving her permission to do it and two, I was not committing to my marriage. As someone said on a post to me, I had one foot in the marriage, the other out, just in case I got hurt.

Instead of worrying about the rollercoaster (I'm there with ya, just as all of us are or have been), focus on these basic facts about you. Something in you set up part of this mess. You need to address that.
By focusing on your need to "fix" you, you can ease the rollercoaster somewhat. Also will go a long way toward helping the marriage.