Clay W. Hunt, a decorated combat vet­eran of Iraq and Afghanistan who became a prominent advocate for troops suffering from post-traumatic stress disorder after he left the Marines, died of a self-inflicted gunshot wound March 31 at his apartment in Sugar Land, Tex. He was 28.

Mr. Hunt separated from the Marines as a corporal in April 2009 and went on to help earthquake victims in Haiti and Chile with Team Rubicon, a humanitarian aid organization founded by a fellow Marine.

“I was able to walk through a rubble-strewn, just destroyed city,” Mr. Hunt said in a Team Rubicon video. “It looked like it had just been carpet-bombed, and I didn’t have to worry about my own safety. I was there to do a job, to help people. And I had a renewed faith in humanity.”

He appeared in a television public-service announcement promoting mental health and awareness sponsored by the Ad Council and Iraq and Afghanistan Veterans of America, a veterans’ support group.

Mr. Hunt also helped lobby Congress on behalf of veterans’ rights and participated in races with Ride2Recovery, a charity organization for wounded veterans.

He was by many accounts a model for helping other service members overcome the invisible wounds of war. But he endured a private battle with post-traumatic stress, depression and survivor’s guilt.

In 2007, Mr. Hunt was serving in Iraq with the 2nd Battalion of the 7th Marine Regiment when his bunkmate was killed in action. In the days after, Mr. Hunt began sleeping in the fallen Marine’s bed, as he later wrote, “to be closer to him.”

About a month later, he was driving a Humvee in Anbar province outside Fallujah when his patrol was ambushed. While the unit was under fire from rocket-propelled grenades, another friend was shot in the throat by a sniper, right before Mr. Hunt’s eyes.

He later told his family that the scene of the mortally wounded Marine being loaded into a helicopter often replayed in his head as he lay in bed, unable to sleep.

Days after the ambush, a sniper’s round missed Mr. Hunt’s head by inches and struck him in the wrist. He was flown to California to be treated and later received a diagnosis of post-traumatic stress disorder.

He recovered and, after becoming a Marine scout sniper, served in the Sangin district in southern Afghanistan in 2008. Two more of his friends were killed.

After Mr. Hunt left the military two years ago, he sought help for his problems with depression and stress. He suffered memory loss and panic attacks and told his parents he had suicidal thoughts, according to a profile of Mr. Hunt that appeared in the Houston Chronicle last week.

He saw multiple doctors and got medication for his mental ailments. But he struggled to get disability payments after his paperwork was misplaced.

“You fight for your country, then come home and have to fight against your own country for the benefits you were promised,” Mr. Hunt told the Los Angeles Times in 2010. “I can track my pizza from Pizza Hut on my BlackBerry, but the VA can’t find my claim for four months.”

In recent months, Mr. Hunt’s life began to crumble.

He had attended classes at Loyola Marymount University in Los Angeles but quit when benefits checks did not arrive on time. A two-year marriage ended in divorce.

“The message I’ve been trying to convey to people is that if this can happen to Clay Hunt, it can happen to anyone,” Paul Rieckhoff, president of Iraq and Afghanistan Veterans of America, told the New York Times last week. “He was involved. He had a supportive family. He was going to the VA. He was doing the right things. And it still happened.”

Clay Warren Hunt was born April 18, 1982, in Houston. After graduating from high school, he attended community college before enlisting in the Marine Corps in 2005.

Survivors include his parents, Susan Selke and Stacy Hunt, both of Houston; grandparents Bill and Muriel Knotts of Huntsville, Tex.; a sister; and four stepsisters.

On his arm, Mr. Hunt had a tattoo that quoted a poem by “Lord of the Rings” author J.R.R. Tolkien: “Not all those who wander are lost.”

In his apartment outside Houston, Mr. Hunt had a shadow box on his wall. In it were pictures of his four deceased friends and his medals, which included the Purple Heart.

“In my mind, he is a casualty of war,” Mr. Hunt’s mother told CNN this week. “But he died here instead of over there. He died as a result of his war experience.”