Georges St-Pierre was in his prime when he retired from the UFC in 2013. He was just 32 at the time, reigned supreme over a stacked welterweight division, and was in the conversation with Anderson Silva as the greatest fighter of all time. Outside the Octagon, however, things were not well.

St-Pierre’s mind and emotions were at a breaking point.

“When I left it was not because of damage [to my body], it was more anxiety, nervousness, I couldn’t sleep well. I kept thinking, it was claustrophobic, just too much pressure,” St-Pierre told the Post on Tuesday.

“When you’re champion, you feel you’re the center of the world. Even though you’re not, it’s an illusion, because of the pressure, and it’s a pressure that is different than other sport because it’s a surviving pressure, your life is threatened in the real, real deepest way … You can die.”

Making matters worse, the UFC didn’t have a legitimate drug testing program at the time and GSP, a longtime advocate for PED testing, felt like he had to fight the system while battling in the sport’s toughest division.

“When I was fighting at welterweight, every time I finish a fight it was a guy, and another guy. The division was the most stuck,” St. Pierre said. “It was crazy, I had killers one after the other boom, boom, boom, and I couldn’t breathe.”

The stress of dealing with PEDs, mental strain and a neverending line of challengers was enough to drive St-Pierre out of the sport. Having defended his title 10 times, he called it quits.

But now he’s back and will fight Michael Bisping for the middleweight title at UFC 217 at Madison Square Garden on Nov. 4. As St-Pierre tells it, part of the reason he’s back is that he is finally free. In the four years out of the UFC, he took time to pursue boyhood passions like paleontology — he’s a huge fan of dinosaurs and lights up when he gets to talk about the survival abilities of cockroaches. He also cleaned up some problems in his personal life and streamlined his fight camp.

Additionally, he feels like the UFC’s drug testing program, which has been overseen by USADA since 2015, took a burden off his shoulders.

“It’s a different set of rules now, I was fighting against a whole system back in the day,” St-Pierre said. “I was very outspoken about the drug problem that we had, people made fun of me, ‘ahhh, he’s a paranoid guy, he says that as a way to go and retire.’ But look what happened now … a lot of the UFC champions have fall to the performance enhancing drugs, getting caught.”

The picture of the future that St-Pierre paints is thus a sunny one. He’s mentally healthier than ever before, USADA is catching fallen champions like Jon Jones, and he doesn’t have to deal with cutting weight now that he’s moved up to middleweight. In fact, the 185-pound middleweight limit is his old walk-around weight, which means that his weight cut should be easier than ever before.

Yet there are storm clouds on the horizon for St-Pierre which he either consciously or subconsciously ignores. For starters, he may say that he’s mentally rejuvenated, but in the next breath, he readily admits that he holds himself to do-or-die standards.

“If I ever lose, I’m retired, it’s finished for me. I’m one fight away from retirement,” St-Pierre said. “I don’t plan on losing but if I do, it’s finished.”

Literally making his fight with Bisping a “win or go home” scenario, GSP further intensifies his situation by limiting his best-case scenarios.

Having hired a nutritionist to help him gain 10 to 15 pounds of muscle — an ironic move since most fighters need nutritionists to shed pounds — GSP says he’s not going back down to welterweight.

That leaves him, just like in 2013, locked into the most stacked division in the UFC. Sitting alongside Bisping is interim champion Robert Whittaker, who won his belt as Bisping waited for the GSP fight, and just below them are Luke Rockhold, Chris Weidman and Anderson Silva. Then there’s Jacare Souza and Yoel Romero, two recent No. 1 contenders who are only down because of recent losses.

It’s a murderer’s row and all eyes will be on GSP as he fights his way through it because the UFC doesn’t have any non-Conor McGregor stars to share the spotlight. Ronda Rousey is gone, Jones is facing a four-year PED ban and the UFC’s old star-making machine hasn’t had any luck polishing up the likes of pound-for-pound king Demetrius Johnson or dominant strawweight queen Joanna Jedrzejczyk.

That leaves GSP all alone until McGregor makes his next move.

No pressure then.