This is a horrible place to be and I'm so sorry. There is hope here though - keep hanging on to that.
Go on to Amazon and buy a book called "Not Just Friends" by Dr. Shirley Glass. This book will be a revelation to you - you will see what has been happening for him (and you) in recent months. You will also be able to see how this happened to you, when you had your EA.
Unfortunately, because you resolved your issues on your own, the reason for it occurring was never dealt with by you as a couple (although I understand why you chose not to tell). Tell us, what was his reaction to your revelation? Complacency works both ways, you know.
When a discovery like this happens, real honesty about what is really going on is rare. He's been confronted and caught out - and so often, people in this situation will admit to only what you can prove. At the moment, you can prove the existence of OW, but you cannot (unless you've seen sexual texts) prove physical infidelity. The script that is usually followed is: nothing has happened (yet), followed by "yes, we have kissed", sometimes "we haven't had full sex" and then, "ok, we have had sex".
When I discovered my H's affair, he was honest with me about the physical activity and the number of meetings, but less honest about the emotional feelings that had underscored the affair. Once you know you've been deceived, it is terribly important to one's own self-esteem that you verify what you're being told. My H showed me some of the later E mails from OW and I had access to his phone bills, plus I'd seen some texts myself, so my mission was to cross match all the irrefutable evidence with what he was telling me himself. In your shoes, I'd do the same.
He's in dangerous territory right now. He thinks he might be in love with OW - he has an emotional connection to her. He is not seeing her in an objective light - and unfortunately, at this "idealising" (of her)stage, any rose-coloured spectacles he's had about you over the years have been removed. He's probably only been seeing your faults - and creating imaginary ones to boot. Men like this are described in the above book as "monogamous infidels". They cannot rationalise having feelings for two people and so they transfer allegiance to the person they feel most connected to at that time. However, so often, these feelings don't last - it is infatuation, not love.
You have several weapons in your armoury here, assuming you want to hang on to him and your marriage. You understand why and how this has happened. I'll bet you have already worked out yourself that this woman is fulfilling a need in your H to be adored and respected. Anything can happen now - it can be a wake-up call for you both to revive your marriage and affair-proof it for the future, it can be a chance for you to call time on a marriage that hasn't been working for you either, or he might leave and convince himself (ridiculously) that this is true love with OW and he'd be better off with her.
The only thing you can control is your behaviour. If you both want to make a go of it, he must sever contact with OW (and that does mean not working with her) and you start a long and painful (but rewarding) journey. If he's dithering, take control and ask him to move out temporarily - this will force him to confront the choices he's got and some of the fantasy element will disappear when he has to explain to his head-teacher, who will have heard the staff-room gossip. Work out what you want and tell him.
You will feel resentment that he has done this to you just when you've been facing other setbacks in your life - I was the same, but people involved in affairs are supremely selfish and sometimes, the timing of them is the least of their worries.
Counselling probably won't work at the moment, while he is so conflicted.