I have to say ppl, especially women say they're blindsided, but if the intimacy and affection is a distant memory and you're just plodding along, there was a problem.
My EX SIL tried to say she was blindsided. What really blindsided her, is that my DB said he reacted a divorce after many years of raising the issues with her.
She took the view that nobody in our family was divorced and whatever the problems, he wouldn't want a divorce. So when he said there was a problem, her response was let's get divorced then. That scared him, until it didn't.. the next time he mentioned an issue, she said let's get divorced if you're not happy he said okay, let's do that.
She never wanted to work on the issues and just expected him to put up and shut up.. she wanted to stay married.
She couldn't believe it when he said he wanted a divorce and she was the one dragging her feet and delaying in signing the papers.
Acting blindsided, when there were issues is sympathy seeking IMO.
I'm not saying some ppl are not genuinely blindsided. My friend was one of them.
They had sex the night before, then the following afternoon, he says he's not in love with her and is leaving. After 25 years of marriage, that's how he ended it.
Another woman emerged within a month. Of course this woman was always there, but my friend is in denial. I think she's knows he was cheating, but it's probably too painful to say.