IGNORE ALL PREVIOUS FAILURE.
This is what you do.
1) Understand that she cheated on you because she doesn't respect you. She thinks you're unworthy.
2) Begin treating her coldly, but not inhumanely. Don't be eager to please. Don't remember stupid anniversaries. Don't act cutesy and lovey with her. Don't call her so frequently. Flirt (a little) with her friends. Fuck her; DO NOT make love (try to get her to go home soon afterwards).
***BE CAREFUL THIS IS A THIN LINE AND MUST BE TRODDEN CAREFULLY. NOT TOO HARSH- NOT TOO NICE. ERR ON THE SIDE OF HARSHNESS.
3) The undoubtable outcome of this, done correctly, is that in 2-3 weeks she will be batshit insane in love with you. Head over heels, besotted.
4) Now you strike. No fancy fuckaroos like the above. Just invite her round one day (act nice and sweet on the phone). When there, after a little while, do this:
-Grab her by the hair
-Calmly and quietly explain to her that you know she fucked Mr X. That she's a worthless, cheap slut. That these will be the final words she ever hears you speak.
-Give her the bum's rush to the door
-Throw her out without another word
-Throw all her shit out with her
-Close door.
Epic revenge. Not only will she be hung up on you for the rest of her life (open to fucking whenever you like) but she'll also go through life burdened by the certain knowledge of her sluttery.
Here endeth the lesson.