Realization itself is a pain, if presented as such. The pain of losing someone you love hurts because emotionally, you're still attached while mentally you know you have to force yourself to move on. There are many different types of break ups, and while some people can split appropriately, more often than not one of the two will be hurting a lot more than their former partner. It happens.
Nothing really compares to discovering the one you love seeks attention elsewhere. This could be the consequence of a break up, but it can also be because of adultery. In any case, it leaves the discovering partner debilitated inside, to know you aren't "special" in that way anymore.
An infinite amount of possibilities are possible to how one would act after realizing such a thing, I'm going to try to discuss my side.
It propels one to accelerate the process of acceptance and endurance because it's there, you can see it. You can stop living in delusion, as if reconciliation was possible. The day you dread, the day when "he" becomes "him" and references to "you" die out like a Dodo. The day of reflection, the day you seek some type of understanding, explanation, but there isn't one. You no longer feel connected with that person that you used to pour your heart to daily, it's gone, losing that emotional connection would put a strain on anyone.
Hasn't been the first, but it may certainly be the last.