This is an overall practice and approach to writing (and life) that is perhaps the hardest, but most important thing I have learned in my twelve-plus years plugging away at the blank page. It's ironic that it seems as though the only way to really achieve success is to put your energy into continuously bettering yourself, your craft and your work, and trusting that the rest will take care of itself.
This not from the perspective of one who has mastered it (far from it), and preaches "perfection" (which is impossible), but as one who has learned this lesson over and over, and continues to see it as a best practice.
A writer’s job is to work on their writing until it is viable in the marketplace. When it is, there is no stopping it - doors open when they didn't before. When it doesn't, there is little one can do to successfully "market" it.
Of course, it’s hard to know when we're ready, or how close we are. There are no hard and fast rules on this - it is all subjective. I think that writers tend to underestimate the amount of continuous forward motion that is required for any project (and ourselves) to be "viable," and focus instead on trying to market what we've done - to see what we can "get," if you will. I believe our energies are always better expended on diligent creative progress - with professional feedback and guidance, if possible.
We all struggle with this (self included) - no matter how many years we've been doing it. We're focused on getting the sale, getting the positive reaction, getting our agent to do something, etc. Getting, getting, getting tends to be our obsession. But more focus on getting almost never seems to have the desired effect.
However, continuous focus on giving - as in bettering and improving what you're offering to the world, staying upbeat and open, never giving up, seeking to grow and serve – is, I believe, a winning approach. I'm not saying don't try to move your career forward. I suggest taking every step that seems right to you at the time, especially if you can do it in a positive way - be it query letters, contests, pitch fests, etc. My point is that the real business of building a writing career is not about that. It's about the writing, the craft, the creative process, and your own growth; so that what you have to give is something others find huge value in.