David Cassidy explains to ET why he was so upset in a viral video showing him complaining about a concertgoer who ignored his ailments during a show last week.

"I could've fallen off the front of the stage because I couldn't see," Cassidy tells ET about the incident. "I got to a point where I was really angry."

The 66-year-old former Partridge Family star was performing at Hudson Valley Theater in Peekskill, New York, on Oct. 28 when a fan shined a cellphone light in his eyes.

"Turn it off and turn it out. Now," Cassidy pleaded with the fan in the video. "Please have an usher remove this person ... I can't take it. I'm blinded already."

Cassidy has had trouble with his eyes for decades.

"I am highly sensitive. This nice light is extremely bright for me," says Cassidy, who has had five eye surgeries, with the first dating back to 1962. "I can hardly see your face. I mean that."

WATCH: David Cassidy Cited for Alleged Hit-and-Run Involvement

The controversial video was just the latest piece of negative publicity to come out against the star over his 48-year career. In 1994, he admitted to ET that he struggled with substance abuse.

"I was a closet alcoholic and I would sit alone home and drink," he said at the time.

Then it was his two DUI arrests -- one in 2010 and a second in 2013.

Cassidy told ET that losing the limelight was part of what drove him into a spiral.

"Suddenly the world that I knew was gone," he explained. "I sat in my room and said, 'I'm just going to sit here until something comes to me.'"

READ: David Cassidy's Stint in Rehab

These troubles culminated in a reckless driving conviction in 2015, but Cassidy assures ET that he's now sober and back at work.

"I still have a job to do," says Cassidy, who is currently on tour across the U.S. through next year. "That makes me want to continue to inspire people."

Cassidy says he feels "grateful" that he now has his disease "under control."

In addition to hitting the road, Cassidy has also been hitting the studio. His new collection of Christmas songs, A DC Xmas, is available now at www.davidcassidymusic.com, and he opened up about finally doing a holiday project.

WATCH: Meet The Band that Inspired 'The Partridge Family'

"Fans have asked me to do it, but it was like I was doing something else. It's like, 'Yeah, I'll do it someday,'" Cassidy says. "Well someday has come and it's so great. There is an original song on it also and it is such a great thrill for me to play and work with musicians. I get to work with the best in the world, and they are like my second family. We travel around the world now. They are off the hook!"