When 12 students and one teacher were gunned down in a mass shooting at Columbine High School in 1999, the tragic event marked a turning point for America. Twenty five years later, some survivors recall how they have dealt with the physical and mental impact of PTSD, anxiety and depression. "It's a big part of my identity and who I am and not necessarily that day, but more who I've become," Heather Martin, a Columbine survivor, told ABC News Live.