Quote: Me holding this affair against Kev because "he could've prevented it" Would be similar to me holding what's wrong with Kiya against God because "he could've prevented it".
With that realization, you "got it" then you proceeded to talk yourself out of it. Like I've been saying, until you want to be happy more than you want to wallow in the sorrow of it all, and the entitlement you feel to be angry/sad/victimized/powerless/etc, you WILL NOT BE happy.
You can't keep doing this. I KNOW you get it, it's just that you can't let go of the security your negative emotions provide you. Yes, I said security. Right now, you feel safe in being angry and sad. If you decided today to be happy, all of a sudden you're in uncharted waters and you'd have to accept the unknown. You KNOW how to be angry and upset, that is natural to you now. Learn how to accept that the unknown is NOT something to be feared or avoided.
Embrace that idea and venture out of your comfort zone. Try to let go of the anger long enough to see that you are no more entitled to it than your H is entitled to his affair. It's something you choose to do because you won't let yourself see the other paths you can walk down. Choose something different today.