Quotes for self-doubt, and the confidence on the other side of it
The loudest critic most people have is the one that sounds like them. These lines won't switch it off. They might turn it down a little, and remind you that the people you admire heard the same voice.

For the voice in your head
The most important conversation is the conversation you have with yourself each day.
James Clear, 3-2-1 Newsletter
We often think we can't do something when really we can.
Leo Babauta, Zen Habits
It's crazy how 1,000 people can compliment you and you'll spend all day thinking about the one person who criticized you.
James Clear, 3-2-1 Newsletter
Feel compliments as deeply as you feel insults.
James Clear, 3-2-1 Newsletter
Try it with the next compliment you get. Don't wave it off. Let it land for as long as you'd let a criticism land. It feels strange, and a bit like cheating, which says a lot about the usual balance.
For being bad at something new
Most self-doubt shows up when you start something. Of course it does. You're not good at it yet, and that's the only way anybody has ever started.
When we watch a child trying to walk, we see its countless failures; its successes are but few.
Rabindranath Tagore, Sadhana: The Realisation of Life
And nobody watching a toddler thinks they should give up on walking.
There's no shame in starting where you are.
Steve Pavlina, Self-Discipline
Every skill you have today was once unknown to you. The human brain is a learning machine. Stick with it.
James Clear, 3-2-1 Newsletter
Never mind being bashful; it won't last long if you keep going.
Louisa May Alcott, Little Women
If you're not embarrassed by what you thought last year, you need to learn more and faster.
Derek Sivers, How to Live
For remembering the greats doubted too
All that I have written so far is rubbish in comparison with what I should like to write and should write with rapture.
Anton Chekhov, Letters of Anton Chekhov to His Family and Friends
This is Chekhov, in a private letter, about work that people still read more than a century later. The doubt doesn't go away when you get good. It just stops being in charge.
The greatest straightness seems crooked. The most valued skill seems like clumsiness. The greatest speech seems full of stammers.
Lao Tzu, Tao Te Ching
Or, in modern terms: the thing you think is obviously clumsy about your work is often invisible to everyone else.
For when you're comparing
Comparison is where a lot of self-doubt gets its fuel. You're setting your behind-the-scenes against everyone else's best moments, and you lose every time.
Don't compare yourself to other people. It won't help. You'll only find what you expect to find.
Steve Pavlina, Self-Discipline
Someone else living a good life does not prevent you from living a good life. There are many ways to win and plenty of space.
James Clear, 3-2-1 Newsletter
Those who try to outshine others dim their own light.
Lao Tzu, Tao Te Ching
No two of us have identical difficulties, nor should we be expected to work out identical solutions.
William James, The Varieties of Religious Experience
For trusting yourself

Self-trust is the first secret of success.
Ralph Waldo Emerson
No one knows better than you what is worth an effort.
Henry James, The Portrait of a Lady
The only limit to our realization of tomorrow will be our doubts of today.
Franklin D. Roosevelt, Jefferson Day address
Roosevelt wrote this for a speech he never gave. He died the day before it was due to be delivered in April 1945, and it was among the last things he wrote.
Everything you need is within you.
Julian Gough, End Poem
After the quote
Self-doubt is loud, but it's rarely specific. Next time it starts up, try asking it for evidence. "I'm going to be terrible at this" becomes "I haven't done this before," which is true, and much less frightening. Then do a small version of the thing anyway. Doubt fades faster in the face of something you've actually done than in the face of any argument.
Dailies has topics built around this: Turn down the self-doubt, You're allowed to be a beginner, Stop comparing and Trust your own judgment. Follow one, and a line from it shows up now and then, often at the moment you needed it.
Questions
What is a good quote for self-doubt?
Leo Babauta's "We often think we can't do something when really we can" is simple and true. For creative work, Chekhov's admission that everything he'd written so far was rubbish compared with what he wanted to write is a reminder that even great writers doubted themselves.
Who said "Self-trust is the first secret of success"?
Ralph Waldo Emerson, the American essayist. The line comes from his writing on success and is often quoted on its own.
How do I stop doubting myself?
You probably won't stop it entirely, and you don't need to. Ask the doubt for evidence, notice when you're comparing your worst moments to other people's best, and act on a small version of the thing you doubt you can do. Evidence from doing beats argument.
Is self-doubt normal?
Very. Many accomplished people describe feeling like a fraud, a pattern often called imposter syndrome. Self-doubt is especially common when you're learning something new, which is also a sign that you're growing.