There may or may not be an OW; I don't think it's helpful to state so authoritatively that there IS one, nobody knows, not even the OP.
The issue is that her husband is behaving abominably, rubbishing his marriage and his family the sooner he's gone, the better.
Whatonearth2020, I agree with ShebaShimmyShake that your husband is lying to himself that he was unhappy - for 15 years? Not a chance. Nobody would allow themselves to stick around for 15 years, having children they didn't want, buying homes they didn't care for and generally wasting their lives drifting along. No. That wasn't what happened.
What's happened is that now he has decided he doesn't want it. He's backtracking to make the leaving sound reasonable. It's not, it's cruel. He could have just said to you that he wanted the marriage to end without dishing up this nasty spite on top. Take him at his word, stop talking to him about it, gather your paperwork and see a solicitor. Tell him nothing.
Stop doing his laundry and stop cooking for him. No need to discuss anything further - the solicitor can sort out the finances and, when he's decided that he does want to see his children, you can take it from there. Mediation can be helpful.
For now though, just take a breath. Do you have a friend or relative who can support you a bit whilst you start coming to terms with this?