No matter who you are, there's no way that everything you write will be publishable. There are lots of reasons:
Maybe you don't yet have the skill level
Maybe this particular story just isn't working
Maybe someone else just published something that's too similar
Maybe there's nothing wrong with the story, but there's no market
Et cetera and so forth
But I don't think everything we write needs to be publishable.
1. Hone your talents,sharpen your skills
I hear the argument all the time about whether writing can be taught. My opinion: it can and it can't.
Skills are teachable. You can teach grammar and punctuation and even a certain amount of eloquence. You can't teach talent. Talent is a natural ability that gives one an advantage over others for no visible reason. It's why I could practice every waking hour and never run as fast as Usain Bolt, or sing as high as Mariah Carey.
But talent needs to be honed. For a writer, that means writing. You can spend forever researching, reading craft books, learning grammar, reading fiction in and out of your genre, but your writing only gets better when you write.
The beauty of honing your talents is that your skills come along for the ride. All the things you've learnt will benefit from being practised and eventually become second nature.
If you decide beforehand that you may not aim for publication, then you may find yourself writing with a certain freedom.
Yes, yes, I know they say not to worry on the first draft and just write. But some (=most or probably all) of us worry from the get-go about sale-ability. If you don't think you'll publish a project, that's the perfect time to try out all those things you've wondered about: 1st/2nd/3rd person points of view, present tense writing, a topic that's been calling you.
3. Lessons learned
You learn from every book you write. You learn what works and what doesn't. You learn what you like and what you don't. You learn what you do well and what you need to work on. And you find your voice.
I've never queried. I've never even completed a second draft. (Shame! As soon as I finish the life-draining novel I'm working on right now, I'm headed into Rewrite Land with a previous MS.) But I've learned what not to do with every novel. How to unsuccesfully intertwine plots. How insanely boring it is to just follow a character around while he does nothing. How monotonous it is when characters use the same gestures every 12 lines. How not to make the last line of your novel, "You will pay!" (Man, I wish it was a lie. See why I never talk about MS2? lol)
But I've also learned a stack of great things. I'm good at voice and character. My plots are nonexistent or derivative. I have to work on setting, since I don't naturally notice visual details. I need to think more about how to build worlds that aren't familiar to the audience (Barbados and Japan). I've learned the art of ending a chapter with an uncontrived cliffhanger (although the same doesn't apply to the end of novels. Poor MS2.)
When all is said and done, every novel that you don't publish isn't wasted. Every book you don't publish makes the ones you do, better.
Thanks for the comments on yesterday's voice post. It's something I think about a lot. According to my critiquers I nail it, and I know lots of people have trouble with it. (Don't worry, what I gain in voice, I lose miserably in plot.) And the more I tried to define it, the more I just kept thinking it's like explaining why you love someone or something. No amount of technicalities can cover it. Which is why explanatory posts on voice often end up sounding like "it just IS", and why in the end, I decided to use poetry, a medium often as elusive as the topic.