The bullet ripped through Frey's right shoulder, shattering the bones in his arm and causing major nerve damage.
But when he got home, he found his mental wounds were at least as debilitating as the physical ones. He hopes the virtual reality treatments will help him get his life back on track.
"It gets me to control my breathing," Frey said. "I just stay in the moment and know I'm not over there."
Frey is experiencing a digital landscape developed by Dr. Skip Rizzo and his team at the University of Southern California's Institute for Creative Technologies. Rizzo and his colleagues created the simulation by recycling the virtual world designed for the combat video game "Full Spectrum Warrior."
A psychologist, Rizzo believes virtual reality therapy is so effective because it forces patients to recall painful memories rather than avoid them.
In traditional talk therapy, Rizzo said, "you're asking somebody who's been traumatized by an environment to imagine it in the great detail needed to produce a therapeutic effect. With virtual reality, we know what the person is seeing."
"Sound is very important for emotion," according to Rizzo. "The sound of a bullet flying by can raise someone's arousal level in a way that maybe a visual might not."
Future upgrades will include smells like gasoline, burning rubber and gunpowder -- important because smell is so tightly linked to memory and emotion.
Rizzo and other psychologists hope the video game pedigree of their simulation will appeal to a digital generation, breaking the stigma of getting help.
"Soldiers don't want to appear weak," said Rizzo. "To admit you have PTSD may be perceived by others -- commanding officers or the people you command -- as a sign of weakness."
Rizzo said this generation "grew up digital. They're used to this technology, and they can look at it as another type of training."
"It's like a video game to them at first," Spira added. "And then after awhile, it makes it easier for them to talk about the emotional components and integrating back with their family."
Frey agrees. "It's something cool. To a Marine, if something's cool, we're all about it."
Virtual reality therapy is also finding wider use in the civilian world.
Stephen King is a retired New York City fire chief who was at the World Trade Center on Sept. 11, 2001. He was so traumatized by what he saw there that he could no longer come into Manhattan after the attacks, unable to drive over the Brooklyn Bridge.
"After 9/11," King said, "I was afraid that that bridge was going to get blown up when I was halfway over it."
He tried traditional therapy and medication for about a year, but says there came a point where he wasn't getting any better. That's when King turned to virtual reality therapy under the watch of psychologist JoAnn Difede, director of the Program for Anxiety and Traumatic Stress Studies at New York Presbyterian Hospital's Weill Cornell Medical Center.
King was gradually exposed to a computer simulation that shows airplanes crashing into the twin towers, smoke rising and the towers collapsing.
"When I first saw it I said, 'This can't help me. This is like a video game,' " the veteran firefighter said. "But it was anything but that. It's funny. I watched this animated scenario, but for me it was the real buildings, the real people that I saw jumping that day."