A lot of new BH posters here are wallowing in that overwhelming desire, common among new BH's, to get their old marriage back. As part of that, they continue to view their WW the way they used to before the A, a sort of idealized, unrealistic view.
You are the obverse of this. That 5 month period while you plotted your confrontation put you in a position where you knew she was lying to you as she left for a tryst with her AP, and lying more when she returned to you. You were able to formulate a clear picture of who she is as a person: a middle-aged woman desperate enough for sexual affirmation that she is willing to lie to her husband to fuck another man. A pathetic individual.
The main sexual organ resides between the ears. One element of R is that the WW must figure out (usually with the help of IC) what was/is broken in her moral compass that would lead her to make these wicked choices in her desire for affirmation, and fix herself, make herself into somebody new, better, somebody you would choose to be married to. Because, let's face it, who would want to be married to a pathetic middle aged woman who chooses to lie to her husband and fuck another man?
That's where your issue is. You don't have sexual desire for your WW, for the person that you see her to be with the clear-eyed vision you gained during those five months. Who would be sexually attracted to such a person? Nobody, really.
In your case, it may never come back. That detail of waiting 5 months, that's rather unusual. A lot of salt was rubbed into your wound by her. The overwhelming vision you must hold is that she is the type of person who would do that.
Some injuries are simply too great to overcome, no matter how heartfelt and earnest the effort. That ephemeral matter of the heart.
My prediction is that desire will never "come back" for you, and why should it? Why would you ever again desire the woman your WW was. It might be possible that, if she transforms herself into somebody you can respect, then perhaps you'll eventually find desire for that new person your WW has become. To that end, as I'm sure you have gathered, MC was a giant mistake. You should discontinue it. Your marriage has no chance unless and until she fixes and improves herself.