Greg Horne of Chatham has written a brave and honest memoir about his struggles with mental illness, specifically obsessive compulsive disorder (OCD).

“I wanted to educate people about OCD,” Horne said during a telephone interview last week. His book, “I Chose Life: A Struggle with OCD, Anxiety, and Depression,” was published in time for International OCD Awareness Week, Oct. 8 to 14.

Horne, now 31, grew up in Chatham as the younger son in a tight-knit family. His father, Roger, was a commercial fisherman whose parents had run the once well-known Horne Cottages on Morris Island. His mother, Suzanne, worked as the K-through-12 speech pathologist in Chatham’s schools. She retired last year. Horne and his brother, Will, who is three years older, were both student athletes, and their grandparents cheered them from the stands at every game.

It was in Chatham High that Horne’s anxious thoughts began, what he dubs “racing thoughts.” He would find himself “going over the same thing over and over again and not having any control over it.”

He grew anxious in ways he didn’t understand. After graduating from Chatham High in 2005, he went off to Winthrop University in Rock Hill, S.C. “That’s where the OCD really started taking over,” Horne says. He became obsessed with thinking that everyone thought he was gay. These thoughts, which had no basis in reality, severely crimped his interactions with his classmates. Not understanding what was wrong, he thought a semester abroad would make him feel better. Yet on his way to Australia he had an anxiety attack at Logan Airport, followed by deep depression in Australia. “Whenever I became sad or depressed my thoughts of being gay ran rampant,” he writes. “On one hand I was positive that I was attracted to girls, on the other hand I had nonstop thoughts telling me I was gay. The OCD was relentless.”

Horne finally alerted his parents to his plight, and he met Will and their father in Hawaii. Realizing he could not return to classes in Australia, he flew back to Chatham. There, he fell into a dark abyss and contemplated suicide. After three days when he was barely unable to leave his chair, his parents drove him to the emergency room, and from there he was taken by ambulance to McLean Hospital’s satellite program in Brockton. His parents followed the ambulance in the family car. “I had the best family support anyone could ask for during this whole process,” he writes.

At McLean, where he began various types of therapy—medication, group talk, one-on-one sessions with psychiatrists—he made friends with another patient, Mandy, also suffering from OCD. “Now, I believed there was a chance I could put this all behind me,” he writes. When Horne and Mandy were both outpatients, they dated.

At the same time that Horne was struggling with his OCD, his father, who had been suffering from headaches, was diagnosed with brain cancer and began treatments. The illness and eventual death of a parent can be enough to disorient anyone, yet Horne coped with this on top of his own treatment. He dedicates “I Chose Life” to his dad, who passed away in October 2009. At that point Horne was “so heavily medicated” that his father’s death did not have the impact on him that it would years later.

“I was walking around like a zombie,” he recalls. “I had to gain a sense of myself before I could grieve my father.”

During the years after his stay at McLean, Horne was prescribed varying doses of various medications as he kept up his therapy. Since he had not yet finished college, during the summer of 2008 he enrolled in classes at the University of Massachusetts, Boston to complete a bachelor’s degree in business and marketing. Although his anxiety increased, he did well. Living off-campus, he took up cooking, and developed a “go-to meal”-- chicken cordon bleu and mashed potatoes. He was taking small steps to embrace life.

After graduating, Horne went on to earn a master’s degree at Bay State College in physical therapy. For several years he has worked as a physical therapist assistant in Brookline.

During these past years Horne’s medications were reduced to the point that he now takes only a low dose of anti-anxiety medication. He has mastered behavioral techniques that help both OCD and anxiety.

Because he was so in the dark when he first developed the symptoms of OCD, he wants to work toward removing the stigma of mental illness and educating others about OCD. His book is a part of that goal. “It’s debilitating,” he says of OCD. “It’s like any other mental disease. Now I wake up with a smile on my face because I know how far I’ve come.”

Recently Horne quit his job, rented out his home in South Boston and bought tickets for the eight-month trip he plans to Thailand, Southeast Asia and New Zealand. When he returns, he will reassess his life and move on from there.

“I Chose Life” is available in paperback and Kindle editions through Amazon.com. For more information on OCD, visit the International OCD Foundation in Boston at www.iocdf.org.