Amy Cuddy is one researcher into this mind-body feedback loop. Her book, Presence has got me practicing sitting up more and rethinking how I work. Really, it's got me thinking about how I live and move in the world.
Her research has come into question, but she also sites the research of others in her book. I'm not sure if all of the research is problematic. But I do know that when I sit up, I feel better. Perhaps I feel better because I am able to bring in more oxygen. Perhaps a shift in perspective helps. Maybe it's the placebo effect. Whatever it is, her book got me thinking even more about how my perceptions and expressions of myself play a role in my success or lack of it.
All this to say that if you're having trouble writing or doing any kind of work in the world, take a look at how you hold your body, check out your thoughts and see if your environment is playing a role. Life is lived from the inside out and from the outside in. Open your mind and your body to new ways and see what happens.