Why I rehearse a conversation that hasn't happened yet
The pattern, in five beats
It's 10:47pm, you're doing the dishes, and you're muttering a sentence to someone who isn't in the room. You start over, change a word, adjust the tone like it matters.
I play out the scene before it happens
It usually starts in the car, or in the shower, or right before you turn off the light. You're preparing what you'll say to your manager about a raise, or to your sister about money she owes you, and your lips move a little without you noticing.
You try a first sentence, it sounds too harsh. You rephrase it, it sounds too soft. You try a third time, just changing the order of the words, as if that will actually change anything once you're standing in front of the person.
Sometimes you go as far as playing both parts. You say your line, then imagine what they'll say back, then prepare your reply to that imagined answer. The whole conversation exists in your head before the real person even knows it's coming.
I'm preparing for their reaction, not for mine
What you're actually rehearsing isn't your own line. It's their reaction to it. You want to know if they'll get angry, if they'll cry, if they'll come back with a remark you won't see coming. So you run through every possible version in advance, so nothing catches you off guard.
The result is that by the time the real conversation happens, you're already tired. You've had it so many times in your head that the real version feels almost unnecessary, a repeat of a repeat. Some of these conversations you never actually have: after playing them out alone enough times, you start treating them as settled.
And then there's the strange moment when the person finally says something, and it's neither the line you prepared for nor the reaction you dreaded. All that work you did doesn't count for anything, except filling the days you spent waiting for the right moment to speak.
What I will not tell you
I won't tell you to just go talk to them without preparing, or that you overthink things, or that you should just say what comes to mind. You already know all that, and it's never once stopped your lips from moving in the car.
The more solid thing is noticing the exact moment it starts: the sentence that comes out on its own in the shower, the scene that replays without you deciding to run it. Not to stop it, just to see that it's there, again, before the real conversation has even had a chance to exist.