The next time
Somewhere in the next few days there'll be a moment like the ones in chapter 1. You'll notice something in a meeting, or see something you could apply for, or finish a piece of work that's ready to go. The doubt will arrive on time, saying what it always says.
You don't need to argue with it. You've read the arguments, and the moment itself is a poor time to rehearse them. Ask it "what exactly?" If it names something, go and check that one thing. If all it has is its opinion of you, then you know where that opinion came from, and you know it hasn't been right very often. Say the sentence. Send the email. Put your name down.
It won't feel good the first time. Your heart will be going, and afterwards you'll probably replay it and find fault with how you put it. That's fine. Write down what you predicted and what happened, and leave it there.
Before you put the book down, do three small things.
Write one plan in the "when, then" form for the step back you make most often, and put it where you'll see it before the situation comes up.
Start the record. Put in three entries from the last year, plain facts, no adjectives. You'll be able to think of three if you don't let yourself explain them away.
Tell one person what you're doing, in a sentence. "I'm trying to speak earlier in meetings." Choose someone who'll be glad, and who might tell you in a month whether they've noticed.
I can't tell you that the doubt will go. For most people it gets quieter in the places where they've tested it and stays loud in the places where they haven't, which is an argument for testing it in more places. What I'm fairly sure of is that you've been working from a poor estimate of yourself. It's been kept low by a habit that stopped the evidence coming in. If you let the evidence in, the estimate will correct itself, slowly.
You knew the answer in that meeting. My guess is that you've known a good many answers over the years, and said very few of them. The next one that comes to you, say.