which I literally didn't even think about, new house, they'll be starting new schools, and a new little brother. [...]Maybe I was being selfish in pressing ahead with the house etc without considering them more in it. [...] I've let them down I think
Trees you're taking a lot of the responsibility on yourself here. And you talk about "sadness."
I know that it's difficult to be angry for you at the moment (milk supply & all that!), so let me be furious on your behalf! You have done nothing wrong - you have been precipitated into this by your WH becoming completely thoughtless and selfish. I am just gasping at the results of his selfishness.
And he can't seem to see what he needs to do, which concerns me even more.
Maybe I'm projecting: I'm in late middle-age now, but like another poster upthread, my parent's marriage was full of contrasts - lots of children, presumably "perfect" normal nuclear family, very family-oriented, picture-postcard white fence, SAHM, etc etc etc.
But as I was in my late teens it all exploded. My father had a brief affair while my mother was away. They didn't separate - there were lots of children, from 7 to 16 years old. And my mother assumed
that my father would have been a dick about money - in those days, divorce settlements were still very judgemental towards women.
But what also became clear was that my father had been looking away from his family - from the family he wanted & created - since I was around 18 months old. If not sexual affairs, he'd been having what I guess we'd nowadays call "emotional affairs" since my parents married. He saw it as having friends and a "life" outside the family, but to my mother, it was a kind of betrayal of the family, and left her to deal with the intensity of family life: looking to a brood of children's physical, material & emotional needs.
My father just kept on checking out of family life. I have sadly come to realise just how this has damaged me. For all the affluence & stability I was raised with (a very comfortable background actually), my deep gut feeling about relationships is panic and bitterness. Because that is what I clearly absorbed (almost with my mother's milk!) from the family dynamic.
Of course, both my parents were weak and damaged people themselves, because of world events. They both lost close family members in the war, and lived in precarious times in that respect.
But the idea of staying together for the stability can be an illusion. It ultimately was in my parents' case. Whether it is in yours is yet to be seen, but as someone says upthread, children pick up stuff.
I could just be massively projecting, so please, take what you need from my POV. And if it makes you respond "I'm not like that!" and reject what I say - well, that also tells you something important.