Adapted from a recent online discussion.
Dear Carolyn: Familiar blueprint: Husband (partner) has affair with married associate at his law firm. I find out, husband comes clean and we work on our marriage. The mistress stays at his firm, and we don't tell anybody. Fast-forward a year, and husband and mistress hook up again, get caught again, and we work harder on our marriage.
This time, the mistress has to leave the firm. But the legal market is tight, and she is the main breadwinner for her family, so it is taking time.
I am having a difficult time watching him go off every day. I feel like telling her spouse would help this. My concern is opening this box, because I really don't know what's in it. I don't want her to get divorced. I really just want her back in her box and out of our daily lives.
AFTER THE AFFAIR
Telling the husband and putting the mistress "back in her box" are a tangible and an intangible thing, respectively, that you feel you can actually do to regain control of your life. The promise in both of them is false, though.
She's out of the box and there's nothing you can do to get rid of her short of ending your marriage.* Stay married, and she - either as a colleague, or as a ghost if she leaves the firm - will remain.
That's not to say you should end your marriage; I'm agnostic on that. I'm merely offering that your post-infidelity reality is gray, and so will demand acceptance of a certain level of uncertainty.
* Even if you do end your marriage, or if the mistress gets hired elsewhere, then you'll just acquire a new kind of gray, since reality is available only in that color. Still, it will be easier without a particular woman's name and face attached to it. Either way, though, it's about making peace with the knowledge you have, versus restoring your life to a point before you had it.
I was about to apologize for the darkness of this answer, but I actually think gray is plenty bright, not to mention easier to live with. Black-and-white comes with expectations just waiting to go unmet; gray is liberating.
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