Identifying Cognitive Errors in Investment Decisions

John is an individual investor who recently sold all of his shares in tech stocks after a significant downturn in the market. Despite a long-term positive trend in the sector, John was influenced by the fear of losing more money. He constantly worries about the recent losses and expressed his emotions about how he could have avoided them. This behavior exemplifies a common cognitive error in investing.

Which cognitive error is John most likely exhibiting?