Anxiety is good, usually. My doubts and fears (“You’ve lost it!” “I knew you couldn’t finish that series!” “You’re old and your mind’s going!” “Young people aren’t interested in the stuff you write about anymore!”) drive me to do the best work I possibly can, because success in the marketplace or other competition is the only way to silence them.
That said… Just about all authors suffer from block once in a while and very often some form of anxiety-- if not most of the time!-- lies at the root. I try to deal with it pretty much the same way as any other anxiety, because block translates itself eventually into a little voice that says “See? You never were a real writer, and the fact that you can’t write now proves it!” I know how to deal with destructive voices-- see above. But during that period between freezing up and the voice manifesting itself, well… To that extent anxiety has won and I’m forced to take a few days off or at least spend them editing.
I’ve also noticed that my quality sometimes deteriorates when I change jobs or there’s a death in the family or even if I buy a car and worry about all the money I just spent. Usually it’s only weeks afterwards, at editing time, that I notice, put two and two together and understand what went wrong. On the other hand I recently wrote some stuffs in the hospital after suffering a heart attack but before being told the damage wasn’t all that serious. I was genuinely worried I might have only a short time left to live. And yet… That came out just fine. So go figure!
I also don’t write worth a damn if I’m really, absolutely deathly ill. But again, I never notice until editing time. Apparently the critical facilities are dampened too.