When a hard moment is already underway, your goal is not to win it. It is to bring the temperature down and protect your person's dignity.
First, regulate yourself. Your calm nervous system is the most powerful tool in the room. A frightened, dysregulated brain reads your face, voice, and body and tunes to them. If you escalate, they escalate. If you steady yourself, you give them something steady to borrow. Lower your voice, soften your face, slow down.
Then redirect rather than confront. Arguing with a dysregulated brain pours fuel on the fire, because in that state your person cannot reason or take in correction. Instead, gently shift attention: a change of room, a familiar activity, a snack, a warm question about something they love. Redirection works with the brain rather than against it.
Reinforce what you want to see more of, which simply means noticing and warmly responding to calm and cooperation when they happen. Connection itself is the most natural reinforcement there is.
Finally, de-escalate with dignity. Do not problem solve or deliver consequences in the heat of the moment. Offer safety and warmth, reduce demands, and wait for the wave to pass. There is a recovery window on the other side, and that is when connection and conversation become possible again.