I am so tired and un-focused in my postdoc. Is quitting my position a good choice?
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.