Robin Williams' depression has been endlessly analyzed in the days following his death. But it's still unclear how much his alcohol and cocaine addiction played a part in his life the last few years. Though Williams was sober during the later years of his life, his previous demons continued to pose a constant struggle.

"Ninety-nine percent of the people who go through that are trying to deal with some pain," the father of three told Parade. "They will say that when they did the drug, they suddenly felt OK. Then, 'I’m not so good. I need to get back [to the drug] and be all right again.' It builds into that cycle."

Robin Williams died on Monday of an apparent suicide by asphyxiation after battling depression. The 63-year-old, who married Susan Schneider in 2011, was found unconscious in his California home at 11:55 a.m. The Oscar winner and father of three was pronounced dead seven minutes later.

Robin Williams, Marsha Garces, Zelda and Cody Williams Robin Williams' children - Zelda, Cody and Zachary - have struggled to cope with their father's tragic death.

Despite the media's emphasis on Parkinson's disease in recent days, Dr. Arnold Washton, executive director and co-founder of Compass Health Group in New York City, says addiction and depression are inextricably linked. "Addiction itself is hard enough to overcome, let alone the double whammy of addiction and severe depression," Dr. Washton told Hollywood Take.

"Suicide rates among people who suffer with the combination of addiction and depression are extraordinary. Depressed mood by itself is a risk factor for suicidal behaviors in the general population and also predicts a greater likelihood of suicide in people with concurrent alcohol or drug abuse. Untreated depression and other mood disorders are major contributors to relapse among people in recovery."

Robin and Zelda Williams When Robin Williams and his daughter, Zelda, posed together at an event in November 2009, their love for one another was palpable.

Williams checked into the Hazelden Addiction Treatment Center in Minnesota in July to maintain his long-term sobriety, but friends close to the comedian believe it was too late for therapists to make a difference. "Just before he checked in [to rehab] it was obvious ... he had not gotten treatment for so long he was too far down the road," a source said.

For the last 20 years, Williams struggled to combat addiction and depression. His biggest inspiration? His three children: Zachary, Cody and Zelda. "The first time I stopped was because my son Zachary was about to be born. I didn’t do rehab or AA. I just stopped," Williams recalled. "Shame. Fear. I was going to have a kid. I didn’t want to be coked out, going, 'Here’s a little switch — Daddy’s going to throw up on you!'"

Robin Williams Robin Williams' wife insists the late actor did not relapse before his sudden death.

Half the battle is finding a level of calm each day explained Dr. Washton, author of Treating Alcohol and Drug Problems in Psychotherapy Practice: Doing What Works. "Williams was in and out of rehab and recovery over the course of many years, perhaps never finding the peace and quiet in his own head that would have allowed him to live comfortably without resorting to the use of brain-modifying chemicals."

But the Mrs. Doubtfire star was determined to try, especially after spending time with John Belushi the night of his cocaine and heroin overdose. “The Belushi tragedy was frightening,” Williams said. “His death scared a whole group of show-business people. It caused a big exodus from drugs. And for me, there was the baby coming. I knew I couldn’t be a father and live that sort of life.”

Zachary was born on April 11, 1983 to Williams and his then-wife Valerie Velardi. From that moment, Williams did everything he could to stay sober. "There were times when many of us were asking questions, saying, 'Hey, what the hell is going on?'" Zachary told The New York Times in 2009. "I’m pretty confident that if he continued drinking, he would not be alive today."

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Arnold M. Washton, Ph.D., is executive director and co-founder of Compass Health Group in New York City, an integrated interdisciplinary team of specialists providing treatment for substance abuse and a variety of behavioral health and medical conditions. An internationally known addiction psychologist and book author, Dr. Washton is one of the addiction field's leading proponents of individualized treatment approaches, as described in his book "Treating Alcohol and Drug Problems in Psychotherapy Practice: Doing What Works." For more information, visit www.compasshealthgroup.com.