Recently I re-read Sam Altman's "How to Be Successful" which he published earlier this year. It was one of the catalysts for leaving my last job (and taking my "sabbatical") and it's something I'd recommend to all (albeit with a pinch of salt).

Under the subheading "Have almost too much self belief", Altman writes:

"Self-belief is immensely powerful. The most successful people I know believe in themselves almost to the point of delusion.

Self-belief must be balanced with self-awareness. I used to hate criticism of any sort and actively avoided it. Now I try to always listen to it with the assumption that it’s true, and then decide if I want to act on it or not. Truth-seeking is hard and often painful, but it is what separates self-belief from self-delusion."

A quality you often hear entrepreneurs talk about is "grit". Where intelligence is largely determined by predetermined factors (genetics, opportunity etc) grit is one that can be learnt or acquired. The following are quotes from books, essays, talks etc around the theme of grit and self-determination.

In "The Anatomy of Determination", Paul Graham writes:

"Being strong-willed is not enough, however. You also have to be hard on yourself. Someone who was strong-willed but self-indulgent would not be called determined. Determination implies your wilfulness is balanced by discipline."

"[Determination] consists of willfulness balanced with discipline, aimed by ambition. And fortunately at least two of these three qualities can be cultivated. You may be able to increase your strength of will somewhat; you can definitely learn self-discipline; and almost everyone is practically malnourished when it comes to ambition."

In "What Goes Wrong", his wife Jessica Livingston says:
"Even though we usually use one word for it, determination is really two separate things: resilience and drive. Resilience keeps you from being pushed backwards. Drive moves you forwards. 

One reason you need resilience in a startup is that you are going to get rejected a lot. Even the most famous startups had surprising amounts of rejection early on."

In an episode of the "Masters of Scale" podcast, Reid Hoffman says:
"The sort of grit you need to scale a business is less reliant on brute force. It’s actually one part determination, one part ingenuity, and one part laziness... You want to minimise friction and find the most effective, most efficient way forward... So forget the tired cliche of running a marathon. You want to be more like Indiana Jones, somersaulting under blades, racing a few steps ahead of a rolling boulder and swinging your whip until you reach your holy grail."
Conversely, in "Grit: The power of passion and perseverance", Angela Lee Duckworth says:
" characteristic emerged as a significant predictor of success. And it wasn't social intelligence. It wasn't good looks, physical health, and it wasn't IQ. It was grit. Grit is passion and perseverance for very long-term goals. 

Grit is having stamina. Grit is sticking with your future, day in, day out, not just for the week, not just for the month, but for years, and working really hard to make that future a reality. Grit is living life like it's a marathon, not a sprint."
In an address to Stanford School of Business students, Sheryl Sandberg said:
"All the stuff out there on grit and determination and working on things that are challenging is all true...

There’s no substitute for hard work. I have a poster in my office that I got from [Starbucks founder] Howard Schultz of two dirty hands. It says the future belongs to those of us still willing to get our hands dirty. Do something you care about and get your hands dirty doing it. You’ll be able to do anything, I promise."
And lastly, in "The Score Takes Care of Itself", Bill Walsh says:
"All successful leaders know where we want to go, figure out a way we believe will get the organization there, and then move forward with absolute determination. We may falter from time to time, but ultimately we are unswerving in moving toward our goal; we will not quit. There is an inner compulsion - obsession - to get it done the way you want it done."