The most important fact for any of us who think of ourselves as writers or artists to remember is that the job people actually pay you for is real. Your employers don't care that you're writing a novel, or a song, or making a movie, or really entertaining your Twitter followers. Or, rather, they care, but not in the positive "we have a real artist working for us!" way you might hope they would.
They care because they think you're goofing off. "Not doing the job you're getting paid to do" is goofing off, even if you're creating great art while doing it. And are you actually creating great art? Be serious now.
Having a job isn't some unique torment you alone suffer. Poke your head up out of your cube and take a look around. See all those other people? They have other things they like to do too. Maybe they play basketball, or take care of an aging parent, or cook, or play in a band. Maybe they don't do much of anything, but whatever it is, coming in to work certainly interferes with it.
But they're all here, and they're all working (probably). So you should be too. Accept that you're one of us, the workers of the world. I can't pretend it isn't sometimes bitter and painful. But it's bitter and painful in a perfectly normal, human way. You'll find you can accept it, achieve some success in your day career (promotions and raises go some way to making acceptance easier), and still get your own work done.