I am so tired and un-focused in my postdoc. Is quitting my position a good choice? About 6 months ago I finished my PhD and started a new postdoc at a prestigious lab at a prestigious university. I was very burned out as I completed my PhD, but I hoped starting fresh I'd find renewed enthusiasm.

However, things started bad and have just gotten worse. I like the university and city and would prefer to keep on for at least a full year, but I'm having a very difficult time concentrating on my work. 

My poor performance stresses me almost more than anything else - after 6 months with little to show for myself, I'm afraid of leaving a bad track record. I feel that my postdoctoral advisers just think I'm confused or stupid, but in truth I'm having a terrible time making myself work.

I have trouble keeping my focus on journal articles and it takes me hours to write short to-do lists just because I keep getting distracted. During research related discussions I tend to "zone-out" and start questioning why I'm even there, totally losing the thread of the conversation.

I know that I am actually a capable mathematician and researcher. I'm not sure what's happened to me, but I know things can't continue like this.
It takes years to get over getting a PhD. It sounds like you are having a more difficult time than some. A therapist might be able to help with the issues. At a minimum, tell your supervisor what is going on. You talk only about your work, but you need to think about how the change of city is effecting you personally. Are you living in the lab, or do you have new friends? Often during the PhD process we drop social activities that we enjoy, it might be time to start these up again.

Now that you have a PhD and a Postdoc, it sounds like you are in it for the long haul. The key is to make sure you balance the personal and work aspects of life.