The reason why people can't let it go, and usually can obsess over the cheating or adulterous behaviour over a period of years, even decades, is because something crucial has changed in their understanding of who their partner is.
You'll know if it's fundamental if someone asked you 'So with this information, would you have gone on a first date with him if you knew he was going to do 'x' to you in future?'
If the answer is 'no', its unlikely you'll ever get over it.
A break in trust, such as cheating, lying or abuse is like a drop of potent poison in an otherwise healthy pool. Washing that out is often impossible and what usually happens is that the person that's been betrayed staggers on in the relationship for a few years before finally accepting they'll never get over it.
Also please know that many men don't want to leave their wives because they want to have a soft, suitable landing pad so they can monkey branch their way from one relationship to another. It's possible he hasn't found his yet, even if he says he regrets cheating.
And, they don't want to be seen doing the leaving. They're cowards and will progressively take the piss in various ways until you decide to take the emotional responsibility on your own shoulders, and finally leave. Be careful that won't be you.