And yet it happens all the time. Are all these cheaters sociopaths? narcissists? What is wrong with people? What was wrong with me that I chose someone capable of this to father my children?
This is the heart of it, isn't it May? I am sure sociopaths and narcissists exist, and I am also sure that many normal people - including you and me - have various personality traits that fall on this spectrum. You chose a human capable of human things to father your children. Someone as imperfect as you. And you don't have to accept those imperfections, and you don't have to stay married to him. But I do think you need to accept that you have no control over him, his healing process, or the past. He will always have done this. That's the work, and it totally stinks and you didn't choose to have to do it. And that stinks too.
Perhaps accepting that will make you feel defeated. Perhaps after a while it will also release you from all the efforts you are making, still, to control or fix or heal your husband or make the past better than it was, or protect your children from having normal humans as parents and perhaps being in the huge number of children who have human divorced parents and who thrive.
I hope this is taken in the right way, but nobody wants to bring their children up in the context of infidelity and divorce but lots of people do and those children are okay. Unless your husband really is a sociopath - and if you think he is, then you're wrong to be in the house with him and your children so I don't believe you really think that - nobody goes into marriage planning to cheat, or planning a SMM, or planning any of the other things that go wrong for all of us. And yet this is what happens. It is common - really common. Why shouldn't it happen to you and your husband when marriage breakdowns happen to half of people who get married?
When you talk about a sacred vow to your children, sometimes it really does come across that you can give your children the happy home you were hoping for by sheer willpower. You can't possibly make a vow that involves the freedom of someone else. You vow to be there for them and to do what is right for them, and you hope your H will do the same. That's all. When you insist you see no signs of your children being affected by what is happening in your household - well - I am sorry to say this and I know it might sound hard (and I mean it in the spirit of wanting to help, not hurt) - but you saw no signs of your husband being in love with someone else for two years either.
I don't think you need to be super human, perfect, or fix this. I do think if you can get past the idea that to accept the reality of the situation is to be defeated, you will stop working on the things that you can't control, and have loads and loads more energy to either work on your marriage, or a happy single life. Maybe if you thought about what you can control, and put all your energies into that, then the defeated feeling will go away?