I agree with those above who say it depends on so many things.
I had a situation sort of like that, but it wasn’t for another lover, but for something else.
I did and do forgive them, but it’s an issue, and it got me to re-interpret the whole relationship. Circumstances are now such that there won’t be a getting back together, but in theory if your scenario happened, it would require cleaning up all the misconceptions of the old relationship, getting that all cleaned up, and then creating new agreements that explicitly were not going to lead to anything like the same misunderstandings or events happening again.