Former Japan and Celtic star midfielder Shunsuke Nakamura said Thursday he hangs up his boots feeling refreshed in the belief he did all he could in a 26-year professional career. A member of Japan’s 2006 and 2010 World Cup squads, Nakamura looked back on that experience during his retirement press conference in Yokohama, just 10 days before the year’s tournament starts in Qatar. “I finished without producing a result, but learning what I lacked there allowed me to adjust my goals,” the 44-year-old said. “In that way I was able to raise my game a little bit after each World Cup.” Nakamura play…