I’ll lead with this: I personally wouldn’t reconcile in your position, but to play devil’s advocate on the point you’re stuck on, let’s get pragmatic. It is clear from what you've described that you were the second choice. You would have to live with that. No amount of soft-pedaling changes the fact that if given the choice, she rather be sleeping with him. She was will to hurt you to sleep with this guy. However, does being "second" actually matter in the long run? Being a backup choice isn't a reflection of your inherent worth; it’s a reflection of someone else’s subjective, and often flawed, judgment at a specific moment in time.
Life doesn’t maintain a permanent leaderboard. The way you enter a situation matters significantly less than how you occupy the space once you’re there. The door is open now, and the opportunity in front of you is functionally identical to what it would have been had you been the first name on the list. Once you do the work, show your character, and build a life, that initial "ranking" evaporates into ancient history. Many of the most successful partnerships and projects started as a "Plan B" but became indispensable.
I'm sure many people who've reconciled have accepted this reality and are still making a good go of it. Though Id suspect many more people chose to rationalize this fact is some way or hide from it.
The real hurdle isn't her ranking—it’s your own internal compass. Can you live with this reality? Can your self-respect accommodate this history, and can your principles bend enough to keep your life from changing, or will the weight of being "second" eventually break the foundation? Take the win if you want it, but only if you can leave the "why" behind.
Personally I would leave her because I don't think a 5 year affair is even in the same ball park as forgivable, like I find even the thought of reconciling under those circumstances to be so inconceivable in line with my own self worth and principles, it would make my head spin... that being said if you're leaving because you were second choice, I'm not sure that's enough.
[This message edited by DRSOOLERS at 12:12 PM, Tuesday, April 7th]