At one level, I've talked before about psychotic persistence [ jlake.com | LiveJournal ]. That's more an explanation of process than motivation, I suppose, but it certainly is related. As I've also previously commented, this stuff is a habit. A tough habit to maintain. Six weeks on the couch watching tv could break it for life.
I also think that question comes from another place inside the asker. We so often wait for permission to do things in life. "I can't get this job until I have these credentials." "I can't ask this person out unless I'm sure they already like me." If you were raised in any of our cultural groups that embraces shame, guilt and/or self-effacement, this stuff is bred into your bones. "Don't put yourself forward" is such a classic meme.
And it's terribly destructive for writers.
All we do is put ourselves forward. By definition. There is nothing in this world more egotistical than believing that total strangers want to hear what you have to say.
There's a lie I used to tell myself when I was drafting a story or a novel. I knew it was a lie, and I didn't care. Call it a mantra or an affirmation or a visualization if you will. It went sort of like this:
"This story is a work of pure literary genius. This story will win awards. This story will be a critical darling and a fan favorite at the same time. This story will be reprinted a dozen times."
These days, I mostly don't care. I think I've internalized that lie, and sometimes even made it true. Because what happens is that I passionately believe in each story as I write it. I live inside the text, the character, the setting, whether for an hour or two, or for months. Once I'm done with a draft, I may chuck it in the dead file after sober reflection, or I may rewrite and send it out. The story becomes a marketing problem, and I quit believing in my invincible genius, for that story. (If I didn't quit believing in my invincible genius, rejections would be torture. And yes, I still get more rejections than acceptances.)
If you deconstruct that lie, what I'm really doing when I'm writing is taking control of my own permissions. I'm giving myself permission to be a genius. I'm also giving myself permission to fail. Those two things are exactly equivalent.
Remember, the best way to learn is through failure. Success is a much less effective teacher. But if you're going to fail, fail big. Petty failures teach petty lessons. Write the Big Idea stories, the grand, sweeping novels. Open your mouth and shout. Be great. Pretty damned good is the failure condition of greatness.
But it starts with giving yourself permission to do it in the first place.
So where do I get my motivation? By putting myself forward. How do I put myself forward? By being motivated. Hell, yes, it's circular logic. Like a merry-go-round, I can step off any time I want to. I just don't want. Like a merry-go-round, you can step on any time you want to.
No one is ever going to give you permission to write but you. No one is ever going to make you good, or great, but you. And on the marketing side, the editorial ninjas will never come to your house and steal your manuscripts for publication. Give yourself permission to write and send out. The motivation will come.
That's my story, and I'm sticking to it. From where do you get your motivation?