Fighting fair means arguing about the problem, not auditing their whole personality.

Arguments expand the moment the subject shifts from a specific problem to a general verdict about the person. After that nothing can be resolved, since nobody has ever won a referendum on their own character. Staying on the actual issue keeps it solvable. Widening the scope usually means you have run out of case.

In the next disagreement, say the one concrete thing that bothered you and nothing past it. Leave out the words always and never. Finish the conversation about the thing you started it about.