It's interesting to wake up and discover you've stopped being superhuman.
There's this thing that happens when you become a pro. You separate into two people -- the one who's visible to the writing world, and the one who gets up in the morning with bedhead and morning breath. That's true in any public role, politicians, actors, what have you, but unlike most of those careers, many of the qualities that make one a good writer often fail to coincide with the optimum qualities for a public person.
We live inside our heads, and those of us who perform live, in public (as matociquala and I both do) also live inside your heads -- our friends, readers, fans, and critics. People develop expectations, which translate back into the way we look at ourselves.
It becomes a career issue. What many writers, and most publishers, want is to create a franchise, something which will hook and keep readers. Think Wheel of Time or A Song of Fire and Ice. That's readership, income, and steady work. At the same time there's a desire, as matociquala says, not to write the same book over and over again.
Some writers do transcend that problem. Terry Pratchett, for example, has built a tremendous franchise with Discworld, while continuing to explore different avenues. Lois McMaster Bujold managed to write every Miles Vorkosigan book in a different style.
It's one of those things that sounds like complaining about winning the lottery. "Help, I'm published and I don't know what to do next!" One the things about this business is that the insecurities and the fears and the internal challenges never end -- like the problems themselves, they just get traded forward.
The cool thing is you can always write another story, draft another book, and keep going. The hard thing is that the world is watching. Once you hit the market and stick around a while, you've lost the comfortable invisibility.