The letter is easy. Sitting across from someone who has been good to you and saying the sentence anyway is the part you keep postponing.
It is the ninety seconds after. "What would it take to keep you?" Or worse, the quiet disappointment of someone who backed you, which is much harder to sit through than anger.
Most people go in with a clean decision and come out having agreed to think about it. Not because they changed their mind, but because the room got warm and saying no twice to a person you like is a skill nobody gets to try in advance. The rehearsal gives you a manager who counteroffers, who takes it personally, and who asks what is really going on.
You say who they are and why you are going
A line about your manager and a line about your reason. Be specific. Vagueness here becomes vagueness in the room.
You talk, out loud, and they push back
Your voice, not typing. They answer in real time and they do not simply accept it. Expect the counteroffer, and expect it to sound reasonable.
You see where you went reactive
A debrief on the specific moments where you softened, over-explained, or left a door open you meant to close. That is the part that carries into the real conversation.
You need a microphone, somewhere you can speak freely, and eight to twelve minutes. Chrome, Edge and Safari can do this. Firefox cannot yet.
Your voice never leaves your device. Your browser turns your speech into text on your own machine. The audio is not uploaded, not stored, and not heard by anybody.
The other person is an AI. Not a recording, and not a person listening in. It answers in a synthesised voice and it will push back.
No signup. Two full rehearsals, free, and nothing to create before you begin.