You rehearse arguments in the car because part of you already knows what you need to say.
STILLNESS
STILLNESS
The imaginary version of a conversation is not just nerves burning fuel. It is drafting. Somewhere in the fourth run through, one sentence keeps coming back in the same words, and that is usually the thing you need to say, stripped of everything you added to make it land harder.
Find that sentence and keep only it. Say it to the person plainly, without the argument you built around it, and stop talking afterwards. Most of the rehearsal was defense against a response you have not heard yet.
What do you already know you need to say to someone?
Free, with nothing to buy inside it.