Look, relationships break up all the time for a million reasons. It's not always because someone has cheated.
But when everything is normal and then one of the couple suddenly says out of nowhere, 'I want to go out more. I shouldn't have had children. I'm not happy. I feel trapped. It's your fault because of XYZ,' then the reason is very likely to be because they've had their head turned, crossed that line and have started something they can't stop.
Then the killer - 'I've felt like this for ages.' The attempt to rewrite history is what really throws you. Because you know the change happened, literally, overnight. Yet they are implying it's been going on for ages. And their deflection works because it throws you off the scent and confuses your ability to understand what has happened.
I'm as sure as I can be from everything you've said that everything was normal until he got together with this woman at a work do. Yes life was hectic (young children are hard work) and you have been suffering from depression which can put pressure on a family BUT he wasn't planning on leaving. No one decides to leave their family, the home they've built, fork out for another place to live / child support, hurt those they love completely out of the blue unless something else is at play. And it's usually the infatuation with a new, exciting relationship.
Don't let him mess with your mind. He will want to protect this new woman / relationship so that nobody blames it for the fact he has left his wife and children. To keep it unsullied in his mind. So he will pretend he's been unhappy for ages. But if that was true you would have known.