“T is what you will,—or will be what you would.”

Variant: It isn't what you have, or who you are, or where you are, or what you are doing that makes you happy or unhappy. It is what you think about.
Source: How to Win Friends and Influence People

Source: His Dark Materials, The Golden Compass (1995), Ch. 18 : Fog and Ice

As reported by Quoteinvestigator on January 11, 2011 http://quoteinvestigator.com/2011/01/11/what-lies-within/ the quote appeared in “Meditations in Wall Street” (1940) by Wall Street trader Henry Stanley Haskins, "a Wall Street trader with a checkered background. The phrase was misattributed because the true author's name was initially withheld. In addition, the assignment of the maxim to a more prestigious individual, e.g., Emerson or Thoreau, made it more attractive and more believable as a nugget of wisdom." Emerson made a number of similar statements — in "The American Scholar," for example, he says "Give me insight into to-day, and you may have the antique and future worlds" — which probably increased the likelihood of misattribution.
Misattributed
Variant: What lies behind us and what lies ahead of us are tiny matters compared to what lies within us.
Variant: What lies behind us and what lies before us are tiny matters compared to what lies within us.

Variant: Success is getting what you want. Happiness is wanting what you get.

Variant: Do what you gotta do so you can do what you wanna do.