She told me every guy always has a plan B in his back pocket. I'm going 100% plan A now.
If she's been unfaithful, she would know based on her experience with another man while having an affair. Think about it, her desire and fantasy was to find someone new & exciting and have a beautiful new relationship with them unfettered by the chains of whatever distasteful history you share together. They hook up, he gets what he wants out of her (sex) and then tells her he's not looking for a relationship or anything serious, her fantasy is shattered by reality, she now feels stupid because she threw away what she had for a chance at something that now won't be anything.
She's projecting on to you, saying that you have a "plan B" is her really saying, I've experienced this before now my karma is catching up with me and I guess this is my lesson for screwing around on you.
For what it's worth, you created mystery, she saw you possibly communicating with other women, pics of women on fb, you stood up for yourself, found your balls, showed her you won't be pushed around anymore and you will move on with your life and make it a good one.
Guess what, you did what you were supposed to do. You didn't plead or beg continuously trying to prove to her that you love her and want another chance to be the "perfect spouse" etc. You took charge, you showed her if thats what she wanted then you were going to go after the same thing with another woman.
Fear of loss.
Crisis.
Her dragging you into the hallway to discuss what you're doing, who you're talking to, etc. All generated by fear of loss & crisis, it's real now, she gets it, you're aren't going to wait for her, if she wants a great marriage, it can't just be about what you can do for her, it's about what she can bring to the table as well.
Don't discount your recent efforts - without them you wouldn't have the positive momentum in the direction you're going in now.
You're not out of the woods yet either, not by a long shot.
The WAW fog is a thick one and not easily dissipated by this one time effort. She'll continue to push, to test you, yes she is insecure, that's the whole purpose behing a woman testing a man, she is insecure, she wants to make sure that the man she's with is strong, masculine and the right choice as her partner.
Making it easy for her now will also prove to her that she can do this again in the future, with little to no consequences so please watch for that. Maintain your self-respect, your boundaries, heck if she pursues sex with you, hold out a bit, tell her that you guys need to be in a better place for sex to be part of your lives again - seriously it works, she will want you even more because of this.
Keep posting updates, you're doing pretty good thus far, a decent example to alot of the LBH's around here ;-)