she has been dating this guy at her work, not just fucking him. she has become emotionally distant from you because her emotions are being placed in the direction of this other man. you cant have sex with the same person over and over without developing feelings eventually. sex isn't just sex when you are in a relationship. when someone pulls it away it is because they are not interested in you. they have lost desire, respect, or feelings for you. you are witnessing the beginning of the end my friend.
women are attracted to men with masculine qualities that are not push overs or submissive. this man at work is being more of the man that she is wanting than you. in many ways you have already damaged her image of you. she doesn't respect you, and has lost feeling for you. the only thing I can see that would work is going the jealousy/ignore route. even that will be a long shot based on your previous behavior.
don't pay her any mind (don't even really talk to her). let her see other women are interested in you, and she will try to get your attention again. either that or go full steam ahead with this other guy. she might do that out of spite, but when you act like "whatever" she will end it. she will feel like she is stuck with him, and she is losing you. if your position is one of i don't care what you do, and she is unworthy of your attention she will not like it. more than likely she will try to get it back. women want what other women want. if no other women are interested in you it suggests you have no sexual market place value. hypergamy will make you lose her. it has already begun.
I would simply let her know that you are going to be going out with so and so you met earlier last week for dinner, maybe a movie, and dancing. if she gets upset just tell her you thought she wanted it this way. play ignorant. tell her you cant call off the date because your date already got her hair done and a new outfit specifically for this. "it would break her heart if you stood her up". the next day keep talking about her. how wonderful she is, how kind and considerate, how great she looked in her new outfit, how much attention she gave you, so easy to talk to, really smart too, and what a great dancer. how the other guys kept asking her to dance and trying to get her attention. how she set up another date for you two next friday.
If you know a very attractive younger woman ask her if she would pretend to be your date in order to win a bet with your wife. tell her that your wife bet you $500 you couldn't get a date for friday night. that women aren't interested in you. tell her you would throw her some coin for her acting abilities. you would only need to get a picture of you guys together, and maybe show up at the door on the day of the date. afterwards she could bail. your wife would be none the wiser. if you don't know anyone, hire a professional ****** to do it. there are plenty of hot women willing to take cash for such an easy task.
relationships are a dangerous game sometimes, and you have to outwit someone's evolutionary inclinations.
it's all fun and games until someone loses an eye.