On Thursday morning, atop the windy Paso del Norte Bridge that connects Ciudad Juárez and downtown El Paso, Tex., 30-year-old Magdalena tries to calm her nerves. It's the closest she and her 10-year-old son have come to being allowed to enter the United States and she's terrified of being turned away again, back to the shelters in Mexico where she and her son, who has a heart condition and needs medical attention, have been living for six months.