Hello,
So sorry you have to be here, but this is definitely the best place to be. you will get excellent advice and support here.
First, copy your post over to 'General Questions II'. There is much more traffic there.
Your situation is serious, but certainly has a chance, if you deal with it forthrightly now. Your WW has had 2 A's, and you enabled them by, as you put it, 'sticking your head in the sand' after you were sure she had the first one.
Arca, I am not bashing you. This is so painful for you, I know that, but you have to get tough and strong now, to fight for what is yours.
We all made mistakes here. I found a suspicious credit card receipt 2 months before D-day. I could have gone to the store to find out why my H spent $200 on a robe for me (that I knew could only have cost about $50). But, I kept telling myself, 'Oh, no. My DH would never betray me. Well, finally, when clues kept dropping into my lap, 2 months later, I confronted, and found out there was a 15-month PA (the $200 bill? He bought me the $50 robe, and $150 was spent on the OW).
So, you know the old saying, 'Today is the first day of the rest of your life'? Well, it is. Start now.
You need to read everything on this website. Buy the Harley books, 'Surviving An Affair' and 'His Needs, Her Needs'.
You have to sit down with WW and tell her you accept your responsibility for your M getting to the point where it was so at risk for an A.
You have to tell her you will work your a$$ off to make it up to her, to make her know how precious she is to you. In effect, you start Plan A.
But, and this is tough, she has to give up OM. now, the prevailing wisdom is that you can't demand that, or make an ultimatum. But, you can tell her how devastating it is to you that she continues the A, and that you expect her to stop seeing OM. You can ask for a NC letter.
If she refuses, you can, and should expose. Is OM M? Expose to his W. Don't know about workplace rules in the UK, but many US businesses have very strict policies about A's between superiors and subordinates. Expose to the employer (although this is an absolute last resort, b/c WW can lose her job, too).
Good luck. Keep coming back. You are in my prayers.