I think this needs a context. Can you give an example?

I guess the most simple answer is to not speak words when you are upset. Count to 10. Excuse yourself by saying you need to be on your own.

And when you notice you say mean things when you are upset, you can apologize for it later.

If you say mean things but you don't notice it yourself, not much you can do, I am afraid. Except trying to be more sensitive to it long-term,