Words you say in a fight stay in the relationship long after the fight leaves.

Arguments end and sentences do not. The specific thing you said at the worst moment gets remembered with precision, often for years, long after everybody has forgotten what the row was even about. The cruel line is usually aimed at something true, which is what makes it land and what makes it stay.

When you can feel it getting sharp, slow down and give up on winning. Say that you need ten minutes, which is almost always available. If something has already been said, apologise for the specific words rather than for the argument, because that is the part still sitting there.