I think it is probably exceedingly rare that someone uproots a long-term marriage without some sort of catalyst (i.e., OW). People generally don't walk away until they feel like they have something better to start walking towards.
I could have written your post at the start of my journey here. 20 years of marriage, never perfect but still considered us a typical, happy family for the most part. Then sudden changes, rewritten history, and H was convinced he was missing out on life out there. Swore up and down that it was nothing to do with an OW or anything like that...just him realizing he wasn't where he was supposed to be. Spoiler: there was an OW.
I hope for your sake that isn't the case in your situation, but it's a common scenario and they almost always deny their hearts out until they can't anymore.
