are gone try and calm down by reminding yourself that getting angry and making a scene is not going to help you in the long run. if you are really angry and feel like you are going to lose control tell the boss you feel sick and need to go home for the day. time out from the problem, even for a few minutes can help you to stay calm and in control.
if possible change the topic of the conversation to something else. it is fine to say “look i am not ready to discuss that right now. i will think about it and we can talk again tomorrow” then 