Infidelity is wildly unfair. That's sort of the nature of it. Certainly there are details that make each person's specific story worse for them.
Certainly having been devout with opportunity to not be, then being cheated on is one of those details. And as things go, more opportunities present themselves. Before she cheated, at least some part of your motivation to stay devout is the assumption that your wife is also being devout. Your assumption is now completely shattered. So the question you have for yourself is quite fundamental in nature.
Do you want to be devout to your wife regardless of whether or not she is devout to you?
Let's assume you do and you have to live with her uncertainty.
For those who’ve dealt with similar thoughts:
1) How did you process the sense of unfairness without it turning into permanent resentment?
My answer to this is that I'm devout so long as I'm with her, but I'm no longer committed for life. I'm choosing every day to stay in this, and if I don't like it anymore, I have given myself permission to end it.
Essentially, no vow of "forsake all others" is balanced by no vow of "til death do us part" on the other. Big reason I wouldn't do a vow renewal.
2) How do you make peace with the fact that your reality and your partner’s reality were so drastically different?
That's a real mind fuck. But I guess at the end of the day you can't know your Ws's reality. Prisoners of our own minds and whatnot. So as long as her behaviors, words, and actions are sufficient for you to feel safe and loved (for now), then you just sort of have to believe that's enough.
3) Is this something that fades with time, or does it require actively reframing how you see the situation?
I think it's both. Revisiting your framing, then giving yourself and her the time to execute according to that new framing makes this sense of unfairness fade. The other thing you can do is be more selfish. Not in a bad, or relationship hurting way.
But for me, I always made it something like 1)kids 2) marriage 3) myself. After the A, I've swapped marriage and self. Phrases like "happy wife, happy life" make me cringe. Be responsible for your own happiness and if she has to make some sacrifice to that, well, so be it. And it's actually OK if she maintains that order as well, so long as she isn't cheating and lying. But there is no way she gets to put herself first, while I put the marriage first.
4) Is this something I should explore before R by dating others?
Personally, if I did date others, I don't think I would ever return to my M. I think maintaining my coparent as a potential romantic option while dating others wouldn't be giving new potential partners a fair shot.
I’m not looking to justify revenge or level the playing field. I’m trying to figure out how to let go of this specific piece, the imbalance, because it feels like one of the biggest things keeping me stuck.
Appreciate any perspective from people who’ve been here.
"Level the playing field" I think is better than revenge. And not in a way that lowers your moral standards to hers. Just in a way that balances your expectations of marriage with hers, including whatever improvements she is making which is measured against her history of deceit and betrayal.
I also recognize my solution works for me, and might not work for you.