Most friendships survive the fight. It’s the one sentence aimed at the soft spot that ends them.

Arguments between people who like each other are survivable almost by default, because the disagreement is about something. What does not get repaired is the line that went for the thing they told you in confidence, the insecurity only you knew about. That sentence proves you were storing it, which is different damage from being angry.

In the next bad moment, keep to what happened rather than what you know about them. If you have already said that sort of thing, name it later rather than apologising for the fight in general. And notice that going for the soft spot usually means you had run out of argument.