Roman Emperor Marcus Aurelius probably wasn’t thinking about writing when he said this:
Do not disturb thyself by thinking of the whole of thy life. Let not thy thoughts at once embrace all the
various troubles which thou mayest expect to befall thee: but on every occasion ask thyself, What is there in this which is intolerable and past bearing? For thou wilt be ashamed to confess. In the next place remember that neither the future nor the past pains thee, but only the present. But this is reduced to a very little, if thou only circumscribest it, and chidest thy mind, if it is unable to hold out against even this.
But I’ve always found a sentiment like this to be enormously helpful in writing. Breaking down a task into little bits, 
isolating them, then doing that one task, and not thinking about all you have to do to finish your project. Even if they’re not done in what would seem obvious chronological order, it’s better to focus on small, doable writing tasks.
When I’m on a larger writing project — as I am now, with a book I’m putting together on heroin and prescription painkillers, I usually spend a lot of time writing what I call “chunks.” Could be anecdotes, or stories shaped around a quote, or just observations or descriptions of a place or person — things that might well make it into the final draft of what I’m writing.
I was talking to a prison inmate the other day who wants to write a book about his life. I said, don’t set out to write a book. It’s like climbing a mountain. Try crossing the street — write a story from your childhood. Just one. then write another, maybe from adulthood. Next day, another. Never think you’re heading toward assembling a book. Pretty soon you’ll have a selection of pieces and can gather energy and encouragement from that.
Here’s what an author at The Atlantic had to say about Marcus Aurelius’s quote.
