Let's say a Soviet exchange student back in the '70s would go back and tell the KGB about people and places and things that he'd seen and done and been involved with. This is not really espionage; there's no betrayal of trust.
There was too much noise. Sirens from police cars and ambulances. Shouts from the crowd on the street eighteen floors below. Traffic from other streets and all of the noises of San Francisco. Mostly, though, there were the voices. Whispering to him. Reminding him of the dark things he had done - all of the little things he had forgotten, all of the big things he had tried to forget. Mostly they reminded him of his biggest secret, a betrayal of trust and friendship long ago. He squeezed his eyes shut as if that could somehow keep the voices away.
To my fancy, one looks back on life, it has only two responsibilities, which include all the others: one is the bringing of new life into existence; the other, educating it after it is brought in. All betrayals of trust result from these original sins.
Let's call cheating what it is: a complete betrayal of trust. Cheaters are people who have a lot of stuff to work out and they're working it out on your time and with your heart. Some cheaters might give you an excuse, some might not have one at all, some might even blame you. No one can tell you exactly what to do when faced with this very complicated and painful situation. But the bottom line is, is this what you had hoped for in a relationship?