DW and i have both hurt each other in different ways (me emotional and her physical), over 2 separate periods.
We have been together since we were 17, and have done a huge amount of growing up over the intervening 25 years or so.
In both instances, the hurt one agreed to forgive/move on.
As PPs have said, this was dependent on complete honesty, marriage counselling (albeit short-lived), absolute transparency, and absolute commitment on both sides.
It's been extremely hard at times, and although I think we are both in a very good place now, I am under no illusion that DW may have decided to herself that once the DCs leave home, that she'll be off.
That is a risk I have been, and continue to be prepared to take.
Neither instance will ever be forgotten by either of us, but (I think) are good reminders of how fragile even special relationships can be, and how much constant work is required to make sure both parties are fulfilled and supported.