Riders of the Apocalypse, book 2: Rage
Jackie Morse Kessler2011

She brought the razor to her inner thighs again and again and again, but with each sting came no release, no comforting numbness that dulled the horror of her life. It wasn't enough. So she cut again - swiftly, mercilessly. Missy didn't mean to cut so deep. But after the party where she was humiliated in front of practically everyone in school, who could blame her for wanting some comfort? Sure, most people don't find comfort in the touch of a razor blade, but Missy always was . . . different. That's why she was chosen to become one of the Four Horsemen Apocalypse: War. Now Missy wields a different kind of blade--a big, brutal sword that can cut down anyone and anything in her path. But it's with this weapon in her hand that Missy learns something that could help her triumph over her own pain: control.