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Sit down. Set a timer for the length of time you think you can tolerate.
When the timer starts, say what you doubt. Out loud. Use plain words. No hedging.
Now sit with it.
When you want to answer the doubt, don't. Sit longer. When you want to reframe it as something productive, don't. When you want to stand up, notice that, and stay.
When the timer ends, do not resolve anything. Stand up. Go do something ordinary. Make tea. Wash a dish. Feed the cat.
The doubt will follow you into the ordinary thing. That's fine. It lives there too.
Do this every day for a week. Do not write about it until the week ends.
On the eighth day, write one sentence. Not about the doubt. About what the room felt like on the third day.