That is why I was so upset last night and today. It would mean the whole of my adult life had been a lie. That I had been delusional. And I really don't think I was, I think he had a chance for some fun and took it.
You can refuse to believe his version of events and end the marriage.
You can rightly judge him harshly for the deception involved in the cheating.
What you did for those years was real and it was honourable.
He chose to kick you in the face in the worst possible way at the counselling session, and you need to take that betrayal you suffered this week as a separate incident of extremely poor choice on the part of your H. While related to the cheating in the past, it is separate and occurred very recently.
You really have to take into account that this man agreed with the marriage counsellor's version in trying to piece together an accurate picture of who exactly this man is that you are dealing with.
That your H is prepared to agree to the idea that your marriage was crap for many years is the current reality of your marriage - this doesn't relate to the past. It is part of the present, an example of the sort of behaviour he is capable of even when he knows his marriage, his family life and that of his children, and his home are all on the line.
Your anger is perfectly valid - the scale of the role you thought you were playing out of necessity now appears not to have been cast in stone; he could perhaps have compromised and the two of you could have shared the parenting more. Worse, you find now that while you were doing all that you did - mucking through inescapable reality day in/night out for the sake of the family - your H was behaving in a disgustingly shallow way and seemingly not at all committed to the vision you though the was committed to.
He snuck around buying me a diamond necklace for christmas, asking advice of my friends, to thank me for being so supportive during a difficult lengthy promotion process, then a few months later started shagging someone else!
I think this is a highly significant incident. I think it shows something about his character that you need to take seriously.
You need to consider this in a different light from the one evident in your post. The timing is not really the issue. The issue is a deeper problem.
The public display of devotion in front of your friends was a calculated performance designed to make them wonder if you were an ungrateful/ paranoid /crazy bitch if you ever confided in them that you had a problem with him. It was designed to make them think he was Mr Besotted and highly unlikely to cheat and on top of that, a man who had succeeded in a long promotion process - maybe a stealth way of attracting the attention of some of your friends too.
This diamond necklace incident is the work of someone who plays and manipulates people, and will not stop.
I strongly suspect that your H was cheating on you for longer than you believe he was, and with more women than you think were involved. He seems to be a man who gets a kick out of manipulating people (see the marriage counselling as an example) and who needs the adulation of women, the thrill of conquest whether emotional or sexual.
You are looking at all the ingredients of a narcissistic serial cheater.