1) Quality is an Emergent Property
An emergent property is one that's intrinsic to a system, and that arises organically as that system operates. Think of relationships: if you try to force them, they wither and die. The best relationships are not forced, but arise naturally as the yield of many interactions.
Quality in writing and other work similarly occurs organically via an effective process, through your numerous interactions with your subject matter. It's the result of your years or decades of preparation--your reading, writing, training, life lessons, etc. In other words, your ability to write well is "part of your DNA." You don't have to force it. And, in fact, you shouldn't force it because...
2) Striving for Quality is Inimical to the Creative Process
If you focus on achieving a quality outcome, you'll likely either:
(a) Succumb to perfectionist terror and get stuck midway through; or
(b) Wind up with a stiff, overworked, and "juiceless" product.
That's why even Gustave Flaubert, notorious for his slavish devotion to perfect prose and "le mot juste," once said, "Success must be a consequence and never a goal." In other words, you should just focus on doing your work as best as you can, and not worry about quality, or any particular result.
"But wait!" You might be thinking. "I do need to achieve certain results! And I do want to do high quality work! So how can you tell me to ignore quality?" And you'd be right: my advice to ignore quality would make no sense at all, were it not for the fact that...
If you want to do great work, you need to do a lot of work. There are at least three reasons for this:
(1) We often need a lot of practice to get to the level of doing great work. (Deprecating the true process of success and career building is perfectionist.)
(2) We often need to warm up, or otherwise prepare, to achieve quality during a work session. (You might need to write five or ten blah pages to get two good ones. And if that sounds bad, consider that, each time he starts a new novel, the prolific Philip Roth writes a hundred or more preliminary pages, from which he feels lucky if he salvages one useful one.)
(3) Aiming for quality fosters perfectionism; aiming for quantity defeats it. From one of the best productivity books ever written, Bayles and Orland's Art & Fear:
"The ceramics teacher announced on opening day that he was dividing the class into two groups. All those on the left side of the studio, he said, would be graded solely on the quantity of work they produced, all those on the right solely on its quality. His procedure was simple: on the final day of class he would bring in his bathroom scales and weigh the work of the "quantity" group: fifty pounds of pots rated an "A", forty pounds a "B", and so on. Those being graded on "quality", however, needed to produce only one pot--albeit a perfect one--to get an "A". Well, came grading time and a curious fact emerged: the works of highest quality were all produced by the group being graded for quantity. It seems that while the "quantity" group was busily churning out piles of work--and learning from their mistakes--the "quality" group had sat theorizing about perfection, and in the end had little more to show for their efforts than grandiose theories and a pile of dead clay."
You need to define it, relative to your project. And you need to have a plan for attaining it, and a way to measure it.
And you need to know why you're striving for that particular kind of quality.
Have you done all that? Too many writers don't. They operate from reflex, or impulse, or a vague stereotype of what they think a writer is supposed to do.
Or, they set a quality goal without considering the real world constraints they face in terms of time and other resources--an example being someone who seeks to write a wildly ambitious novel, or achieve another ambitious goal, in small, odd bits of time. (Big projects require large chunks of time.)
For them, quality is a kind of vague, perfectionist mirage that they will never arrive at--and thus an ongoing source of shame and self-recrimination.
If you define a realistic quality standard for your project, you'll be far more likely to attain it, and far more likely to enjoy your success when it happens.