This is a very difficult situation as each relationship is different.
If your partner has had a fling with someone at work for instance and you know that they are seeing each other daily, even though the fling has finished, it would be very difficult to forgive.
It is the trust that will hit you; you would be thinking if he or she was late from work were they seeing each other again.
You would be quite in order to ask them to leave that work place as a sign that the fling is definitely finished.
Can you forgive them without searching through their pockets and their phone if you have the chance, and checking up on them and do it on a constant basis.
If not it will eat at you and you will find your relationship will be doomed.
You have to be able to forgive and trust your partner. Trust has to be earned and built up, only then will you will able to repair the damage done to your relationship.
However some people in relationships put up with their partner cheating, some cheating regularly.
The reason many don’t leave their relationship is because it just suits them.
Perhaps they are willing to put up with things because they have financial security or status of some kind.
Maybe they feel they can have relationships too.
Others can feel trapped in the relationship and cannot find the momentum to do anything about it, so they just put up with everything.
We are all entitled to be happy, so after living with someone for a while, you find that you cannot forgive them, well the answer is to move on and not waste your life.