it takes a lot of practice as you've discovered!
At the heart of being assertive is the ability to detach, and to remove the emotion from the situation which is often what I do.
So instead of launching into a rant reacting to something to what someone says, I'll pause for a moment, buy myself time, often by listening to what the other person is saying and then after signalling that I've taken on board what they've said, I'll give my point of view. That sounds easy, it isn't but the key thing is to slow things down, which helps to keep emotions in check.
In the case of your situation where you've had no time to prepare it's probably best to handle it a bit like this
Colleague - (snippy, rude) how dare you copy Jane in on that email yesterday, that was completely outside your authority. Don't ever do that again, is that understood?!!!
Zambia - (pause, don't react defensively, look at Colleague steadily) OK I can see you're upset by that. Let's take this outside the meeting and we can talk through it together. (Stay polite, collaborative and calm)
Colleague - I don't need to discuss this any more, just do as I say, and stop arguing back!
Zambia - (still calm - contrast the behaviour of rude colleague) that's fine, I'm happy to discuss it with you later today, let me know if you change your mind.
Key thing is don't apologise or act meakly, don't get into the weeds about trying to justify your decision to copy the email to Jane. That deed is done, if the colleague doesn't wanted to discuss it then that's their decision. Key thing is to diffuse the immediacy of the situation, ,it's their shit their issue so don't allow the person to suck you into a vortex of arguing about the minutiae, mostly it's just them on a power trip!