The Miracle Mile Improvement District chief abruptly resigned under pressure Monday, saying she admitted abusing the district’s credit card, the latest setback for the midtown shopping district.Mimi Nguyen, 31, reportedly said she bought herself about $40,000 worth of personal items with district money over the past three years, said Mike Hakeem, the attorney for the Improvement District.“We accepted her resignation and terminated her Monday,” Hakeem added.