Earliest attribution found in Who Said That?: More than 2,500 Usable Quotes and Illustrations https://books.google.nl/books?id=7mn8AwAAQBAJ&pg=PT63 (1995) by George Sweeting. Online sources always attribute the quote to Augustine, but never specify in which of his works it is to be found.
Disputed
Source: Bird by Bird: Some Instructions on Writing and Life
“Believe what you like, but don't believeyou read without questioning it.”
Source: Questionable Creatures: A Bestiary
Source: Dictionary of Burning Words of Brilliant Writers (1895), p. 189.
“You are what you believe yourself to be.”
Source: The Witch of Portobello (2007), p. 152.
Context: You are what you believe yourself to be.
Don't be like those people who believe in "positive thinking" and tell themselves that they're loved and strong and capable. You don't need to do that because you know it already. And when you doubt it — which happens, I think, quite often at this stage of evolution — do as I suggested. Instead of trying to prove that you're better than you think, just laugh. Laugh at your worries and insecurities. View your anxieties with humor. It will be difficult at first, but you'll gradually get used to it. Now go back and meet all those people who think you know everything. Convince yourself that they're right, because we all know everything, it's merely a question of believing.
Believe.
Source: Horns
“If you don't believe in yourself, then who will believe in you?”
Attributed to Korda in The Power of Choice (2007) by Joyce Guccione, p. 52, the earliest occurrence of such phrasing yet located is by Martin Lawrence, in "What Up?" in Upscale : The Successful Black Magazine (February 1993), p. 79: "If you don't believe in yourself, then who will believe in you? The next man's way of getting there might not necessarily work for me, so I have to create my own ways of getting there."
Disputed
“Relationship Principle 5
Don't believe what anyone tells you about yourself.”
Variant: Don't believe what anyone tells you about yourself.
Source: Why Men Marry Bitches: A Woman's Guide to Winning Her Man's Heart
Variant transcription from "Death of a Genius" in Life Magazine: "Certainly there are things worth believing. I believe in the brotherhood of man and the uniqueness of the individual. But if you ask me to prove what I believe, I can't. You know them to be true but you could spend a whole lifetime without being able to prove them. The mind can proceed only so far upon what it knows and can prove. There comes a point where the mind takes a leap—call it intuition or what you will—and comes out upon a higher plane of knowledge, but can never prove how it got there. All great discoveries have involved such a leap."
Unsourced variant: "The intellect has little to do on the road to discovery. There comes a leap in consciousness, call it intuition or what you will, and the solution comes to you and you do not know how or why. All great discoveries are made in this way." The earliest published version of this variant appears to be The Human Side of Scientists by Ralph Edward Oesper (1975), p. 58 http://books.google.com/books?id=-J0cAQAAIAAJ&q=%22solution+comes+to+you+and+you+do+not+know%22&dq=%22solution+comes+to+you+and+you+do+not+know%22&hl=en, but no source is provided, and the similarity to the "Life Magazine" quote above suggests it's likely a misquote.
Source: Attributed in posthumous publications, Einstein and the Poet (1983), p. 136
