On the 25th anniversary of the 1999 mass shooting at Columbine High School, two mothers and survivors of that tragic day remain haunted by the horrifying memories that comes flooding back as they send their children off to school. Missy Mendo was a 14-year-old freshman and Amy Over was an 18-year-old senior when two teenagers opened fire inside the Littleton, Colo., school on April 20, 1999, killing 12 students and a teacher, and wounding 24 others. At the time, it was the deadliest school shooting in U.S. history. Mendo, now 39, told People.com that memories of the carnage came streaming back…