I've only been in Plan A for a month myself, and it's had only the slightest amount of obvious impact, plus I'm dealing with the fact that I'm not only the BS I'm also a FWS myself.
All I can say is:
Your doubts and fears are pretty typical.
MB's advice seems to go against the status quo and basically TELLS you to BE somewhat of a "doormat" and put up with "cake-eaters" and continue to show love and deal with everything in a loving and calm manner...when all you want to do is punch him in the face, throw him out of the house, and deliver ultimatums to get with the program and get rid of OP or else.
The logic is this - give your WH all the reason he needs to dump you and go with her, and it will happen.
Yell and scream at him, treat him with hostility and anger, and he will have all the ammo he needs to say "yep, see, I'm right to stay with my OP, seeya honey, the D papers will arrive shortly."
He's being very "foggy" right now, torn between two women - you have to show him in a loving yet firm manner what exactly he will lose if he chooses her over you. That means you have to be more loving and attentive and peaceful than you ever have been.
If he doesn't get the picture, then and ONLY then do you revert to Plan B and REALLY show him what he's missing.
Then if he still chooses her...well, you gave it the ol' college try and you can feel justified and righteous in moving on and not looking back while he will spend the rest of his days wracked with self-doubt and wondering what could have been.
The way it was explained to me that worked was this:
Pretend your S has been overtaken by an alien being. Your H, the one you know and love and married is still in there, but this alien force has taken hold of him.
When he's indifferent or hostile to you, when he chooses to talk to/be with the OW, the alien is in control. When he's being sweet and loving and apologetic to you, that's your H talking.
It's your job to coax the H out of hiding and kill the alien with kindness.
Keep in mind that most OP aren't the real deal - they're a farce, a figment of your WS's over-active imagination, a smokescreen for your WH to ignore the real issues and make excuses and justifications. They fill EN's that only you should fill and that only you really CAN fill in any sort of solid and meaningful way.
It's "tough love" if you will - show him how big a mistake he made by showing him what he stands to lose, and make him grateful that you're giving him a 2nd chance.
And mostly importantly GIVE IT TIME AND BE PATIENT.
None of this happens overnight, it often takes months and months.
Again...I don't have real firsthand experience yet, but this is the logic that's been repeated to me by others when I have come here and posted a message similar to yours.
I'm sticking with it, and it seems to be working, slowly but surely.
Read my previous threads and others to get an idea.
AND STICK WITH THE PROGRAM! <img border="0" title="" alt="[Smile]" src="images/icons/smile.gif" />