"I'm probably only one more head knock away from being a vegetable or not being able to play with my kids," the 32-year-old told Fox Sports News on Wednesday.
"After a couple of really big head knocks, I had to make a big decision... I love the rugby game so much but I needed to look after my mental health."
Fainga'a said the after-effects of his injuries were so severe that he needed help standing up during the wedding of his brother and former Wallabies and Reds team mate Saia.
"In 2016, my twin brother got married and at the altar, I was actually getting held up because of the head knocks," he added.
"I received a couple of really big head knocks over my career and I was standing at the altar getting held up, I got walked out by someone."
Fainga'a urged young players suffering from similar issues not to hesitate to quit the game.
"My message would be it's never too early, it's never too late to finish up," he said.
"Everyone loves rugby but it only takes one head knock. The easy decision is to keep playing, the hard choice is to say I'm going to give this up and do something else."
(Reporting by Shrivathsa Sridhar in Bengaluru; Editing by Nick Mulvenney)
