By Michael Rosenberg The U.S. alpine skiing star will leave Beijing without an individual medal. While she’s been incredibly candid, she can’t find an explanation for her struggles. BEIJING — American skier Isabella Wright said, “We’ve all been there,” but the truth is that nobody ever has. There has never been a Winter Olympics performance quite like Mikaela Shiffrin’s. She is the best slalom skier in the world and she can’t finish a slalom. Her last individual race of the Olympics came and went Thursday, and it was like most of the rest: She came and went off the course. She was in fifth pla…