The former Partridge Family singer and actor, 66, told People magazine he was fighting dementia, a disease which his mother also suffered from.
"I was in denial, but a part of me always knew this was coming," Cassidy told People.
Cassidy told the magazine he had decided to stop touring as a musician to concentrate on his health.
"I want to focus on what I am, who I am and how I've been, without any distractions," he said. 'I want to love. I want to enjoy life."
His comments followed videos taken by fans of the singer struggling to remember words to some of his old hits at small venue concerts in southern California on Saturday and Sunday. At one point, he appeared to fall off the side of a small stage before climbing back up.
Cassidy, whose hits "Cherish" and "I Think I Love You" had teenage girls swooning in the 1970s, has struggled with drinking and financial troubles in recent years.
In 2015, he had to auction his Florida home after a bankruptcy filing. He was arrested three times for drunken driving between 2010 and 2014, and was ordered to rehab as part of his sentence in 2014.
Cassidy appeared in several stage shows after his career as a solo singer declined. He played an aging former teen heartthrob in the short-lived 2009 TV comedy "Ruby & the Rockits" and was a member of the "Celebrity Apprentice" reality TV show in 2011.