I'm so sorry he'd done this to you.
You didn't notice because he probably wasn't unhappy, he just found a way of eating his cake and having it too. Many do.
Him not loving you for years, being unhappy & all the stuff about how you are...it's a 'script'. It's 'The Leavers' script, it enables them to blame you & feel justified in leaving. It's all bullshit.
He feels 'free' to leave now your youngest is off to uni. Great huh. Lovely start to adult life for them. Wanker.
As for not being able to leave due to complications with her husband. For Fuck Sake. The laugh will be on him if she doesn't leave her husband. I'd help that along, ring her DH.
I know what you want to do is 'save your narriage' and beg him to stay, do what we call the 'pick me dance' but it doesn't work. Marriages after affairs can only be saved if the one who had the affair is genuinely remorseful and prepared to out in a shed load of work to rebuild the trust. That's not your DH. If he 'changes his mind, realises what he woukd lose, realises he loves you' it will all be bullshit. To will be that she hasn't left her DH. You don't want to be 'fall back'.
I know this will be hard, but pack a bag of his essentials, hand it to him & tell him you don't care where the fuck he stays, but he's no longer welcome in the family home.
Get your ducks in a row regarding bank accounts, paperwork, solicitors etc.
Please don't try to hang onto what you no longer have. It will only make things harder for you, honestly 💐