evega,
I recently picked up a copy of "Survivng an Affair" and there's a sample letter that may help you regarding Plan B. It goes something like

I aplologize for my part in creating an envirinment that helped make your A with X possible..... blah, blah, blah

I'm willing to avoid all mistakes I've made in the past and create a new life for us that will meet your needs, but I cannot do so until you end you relationship with X once and for all.....

Until then, I will avoid seeing you or talking to you. I ask you to respect my decision to separate from you this way. You must know the suffering I've endured because of your relationship with X, and while I still love you, I can't see you under those conditions....

As soon as you are able to completely separate from X, and take measures that ensure permanent separation, I will be willing to discuss our future together....

fill in any blanks and add on as you go.

Evega,

remember that your H is in a fog, and pressure only makes him rebel. The OW is probably not putting him under any pressure to D you and if she is, that could also be a window of opportunity for you.

Plan B is to help your H realize that all of his needs cannot be met by the OW. In order for him to see this, you have to STOP meeting his needs, sit back and let the OW meet them. Right now he's a cake eater and the longer you meet his needs while some of them are being met with OW, you encourage it. He has nothing to lose by staying with her right now. Plan B was designed to put him in a position to reconsider what he may be losing. But once you write and deliver the letter, you MUST follow through with what you've laid down. Refer to the Plan A/Plan B board for support and advice on how to cope with the difficulties of Plan B and look to MB vets for support when you're not sure how to inplement it.

I'm so sorry for what you are going through. D-day for us was the night before our third child was born. I know what it's like to be abandoned by someone you love when you need them the most.

Give the advice you gey here a chance to work. No matter what the outcome, at least you will know you have tried everything and still kept your dignity, sanity and self respect.