In the dark, quiet moments, the constant ringing in his head brings back difficult thoughts.
"That's when the memories start kicking in and you can't sleep because, you know, the whole, 'I shoulda done this, I coulda done that,' " he said. "And then you see the faces of the soldiers that died."
Carter said he believes that one more name should be added to the death toll of the battle of Kamesh: Ed Faulkner Jr., who struggled with drug abuse and mental problems after surviving the battle.
He left the army with severe PTSD and died in September 2010 after a fatal overdose of methadone and Xanax. While there was no evidence of a suicide, Faulkner's friends -- including Carter -- believe his death was linked to the battle.
"I honestly believe that, yes, he was the ninth victim of Combat Outpost Keating," Carter said. "And I also believe that he won't be the last."
Carter also struggles with PTSD, and now has regular therapy.
"I didn't believe it was real until I experienced it," he said of PTSD. "I thought it was just an excuse to get out of duty ... but once it hit me, and I realized it, I was blown away. How could I be so ignorant?"
Carter wants to be a voice to destigmatize these invisible wounds of war, and he hopes that the attention he receives with the Medal of Honor will show that there is nothing weak in seeking help.
"What we need to do is take the first few steps," Carter said. "We need to realize that yes, this is affecting me, and I need to fix this."