"a long-ago relationship that I tried to improve but ended up breaking it off. (He was also not a monster but a bit of a fantasist, always assuming things would work out and overestimating his influence with others.)
He was so shocked and I said we’ve talked about this already, you knew I wasn’t happy. He said something like “Yes but I didn’t know you were serious!” 🧐😀 Some people will only prioritise you after threats and tantrums (if then) and who wants to be in that relationship??"
I think this is particularly prevalent among men, and it reflects a fundamental sexist and disrespectful view of women - ie that women are always moaning, nagging, and emotional. They deal with it by ignoring their wife or tuning her out or pretending to listen while making soothing noises, in the hope that she'll eventually shut up. So these guys are ALWAYS "blindsided" when their wife - who has tried for years to be listened to - leaves. It's so common that divorce lawyers have a term for it: Walk Away Wife.
This pattern also requires a socialised male laziness and complacency towards relationships. A PP above mentioned how her H basically thought that once he'd "got the girl", that was it, he didn't have to work at the relationship anymore. I think this thinking is also extremely common among men, including in otherwise decent men. And on listening to women talking about dating, it's also obviously still very prevalent in the younger generations: the guys perform what they think Women (we're seen as a homogeneous mass) want, they put on this charming and manipulative mask, but once they've tricked their way into their target's bed, affections, and trust, they can relax and "be themselves", much to the bafflement and hurt of the woman.
i think both of these things - disrespect for women and relational laziness - were operating in your ex, OP, and both led directly to this rupture. These are very deeply rooted and heavily reinforced patriarchal attitudes, it takes a lot of active learning and effort and self-control to change them. I don't get the feeling that he's anywhere near that, because what's happening now is him plastering on his charming mask again and launching a new campaign to trick his way back into your life (and house).