"It was when I questioned everything. Like, 'I can't put a shiny face on this. There's no bright side to this, no upside. This is all just regret and pain.'"
Fox feared that he was unable to find the optimism he has maintained over the years and doubted if he was the right person to offer hope to others.
The 'Back to the Future' star told PEOPLE: "Parkinson's, my back, my arm ... it still didn't add up to moving the needle on the misery index compared to what some people go through.
"I thought, 'How can I tell these people, 'Chin up. Look at the bright side. Things are going to be great?' "
Fox opened up how he enjoys living a "quiet life" with wife Tracy Pollan and his children. He and Tracy share son Sam, 31, twins Aquinnah and Schuyler, both 25 and daughter Esme, 19.
He explained: "My life is now quiet, and I'm actually having a really good time.
"People don't believe me, but I love life. I love being with my family. I love being with Tracy. I love that I don't do a lot of useless stuff that I used to do, because I don't have the energy or the time."