Oscar, Golden Globe, Emmy and Grammy-winning entertainer Robin Williams was one of the biggest talents ever to hit Hollywood. But sources tell FOX411 that it was this immense success that contributed to Williams' struggles to live up to his own exceedingly high expectations.

"Robin was very depressed. His TV series got cancelled, his last film 'The Angriest Man in Brooklyn' hardly opened," one insider told FOX411. "He was in a funk."

Williams starred in the 2014 comedy "Angriest Man in Brooklyn" alongside Melissa Leo and Mila Kunis, which was panned by critics and released to a limited number of theaters in May before going to DVD and VOD.

Also in May, CBS announced it had cancelled Williams' highly-anticipated television comeback "The Crazy Ones" after just one season. The sitcom opened strong with 15.5 million viewers, but by its final episode had fallen to 5.2 million.

"That [show] gave him the discipline to show up, to stay strong and keep working," another source said.

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Williams also starred in the Dito Monitel drama "Boulevard," which debuted earlier this year at the Tribeca Film Festival, but has yet to find a distributor.

Another source close to Williams' family said that aside from his professional pressures, William's suffered from survivor's guilt and was never really able to get over the loss of his three dearest friends in the entertainment world: Christopher Reeve, Andy Kaufman and John Belushi.

"Robin outlived them all. He was a sensitive soul who struggled with the unfairness of it all," said the source.

Williams, 63, died on Monday. Sheriff's officials said a preliminary investigation determined the cause of death was suicide due to asphyxia.

The actor was hospitalized in March of 2009 with heart problems, causing him to postpone his one-man comedy tour to replace his aortic valve. The actor struggled with cocaine and alcohol abuse throughout the '70s and '80s, but remained sober for two decades until a relapse in 2006, in which he underwent  rehabilitation.

We’re told that alcoholism in particular was still very much a daily struggle for the Oscar-winning actor. In July, Williams once again checked into a rehab facility, which his rep insisted was just to maintain his health after completing back-to-back projects.

Williams' next movie, "Night at the Museum: Secret of the Tomb," where he reprises his role of Teddy Roosevelt, is set to hit theaters Dec. 19. Although it was not a done deal, a sequel to the 1993 hit "Mrs. Doubtfire" was said to be in the writing stages. According to Variety, any plans for that film will likely now be scrapped.

Veteran Hollywood publicist, Michael Levine -- who knew Williams for 30 years -- noted that while he often talked about his depression publicly in an attempt to help others, he very much suffered in silence.

"Very few people in this world reach the level of fame Robin Williams did and could understand the type of depression he dealt with,” Levine added. “There tends to be a lack of compassion -- 'So what ,you’re famous' -- and it’s hard for people to then empathize. People like Robin often feel like they have to completely isolate themselves from the fishbowl they live in, and are so isolated they are afraid to ask for help."

But Williams didn't fail to show appreciation when others offered their own.

“I’m feeling much better,” the soft spoken comedian told us at his first public appearance in 2009 after undergoing heart surgery. “And thank you very much for asking, it’s nice to know people care."

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