By Jonathan Stempel Feb 25 (Reuters) – Billionaire investor Warren Buffett on Saturday signaled he has lost none of his enduring confidence in the U.S.economy and his company Berkshire Hathaway Inc . In his annual letter to Berkshire shareholders, the 92-year-old Buffett urged investors to focus on the big picture over the long term, rather than higher inflation and other factors that in 2022 dampened stock prices, though not Berkshire’s. He also urged Americans not to be convulsed by „self-criticism and self-doubt,“ saying the country’s dynamism has benefited Berkshire in his 58 years running the company from Omaha, Nebraska, and...