SPRINGFIELD — Brian Nelson’s years in solitary confinement left him terrified of other people, and he says he can still taste the concrete dust from his cell, even though he’s been free since 2010.The 51-year-old is afraid to ride the bus, he takes five psychotropic drugs and sees a psychiatrist every week. Even when he’s at a park surrounded by grass, he says everything starts turning gray and he remembers how tiny air pockets in the walls kicked up dust whenever he would clean his cell at a now …