Pierce made the stunning revelation on the "All The Smoke" podcast Thursday ... saying he was a changed man after he nearly lost his life in Sept. 2000 at a night club in Boston.

"It was so bad that I couldn't even like, sleep," said Pierce, who was 22 years old and just entering his third season with the Celtics at the time ... "I had to have a 24-hour police surveillance in my house, that's how paranoid I was."

He added, "It changes you, dude. You don't know where to go, you don't know who to look at, you on your toes. You're like really on your toes, like, 'Man, I'll kill somebody.'"

Paul was famously attacked at the Buzz Club on Sept. 24, 2000 ... saying he was stabbed three times in the stomach and five times in the back -- all over what he claims was an innocuous conversation with a woman.

Pierce nearly died from the incident ... and says he was so emotionally unstable afterward, his mentality toward people changed for years.

"People don't know this, but I actually carried a gun for two years right after that," Pierce said. "I was so paranoid. Like, I kept it in the car, I had it on me, I was so paranoid after that. I was just like, I couldn't be in crowds. Something like that happens to you, man, it's traumatic."

Pierce says he turned to basketball to help him get through the situation ... claiming after the incident, he did nothing but hoop and stay at home.

"I was like to the point of like, losing it," Paul said.