(NewsNation Now) — Jim McMahon is one of the most iconic quarterbacks to play professional football. Larger than life on the field and off, McMahon led the Chicago Bears to their only Super Bowl title, in 1985.

He’s back in Chicago for his forthcoming documentary about his life, “Mad Mac.”

“I always love coming back to the city,” McMahon told NewsNation. “This town’s always treated me great. If you play hard and win, they’ll love you forever.”

But after 119 games in the NFL, more than half with Chicago, McMahon says he’s experiencing symptoms consistent with CTE, a brain disease hundreds of former football players are fighting. In a 2017 study, 110 of 111 brains donated by former NFL players had the disease.

“My body feels, you know, as good as it’s going to feel after 18-plus surgeries,” McMahon said. “I went back to Medellin, Colombia, two years ago and had 275 million stem cells put in my body.

“My head is really the worst of my problems.”

In 2012 McMahon was diagnosed with early-onset dementia. He said he was “losing (his) mind.”

It began with bad thoughts and headaches, forcing him to lie in a dark room for weeks.

“When these doctors finally told me what was wrong, they said the reason you feel better when you lay down is because gravity is off your head enough so that the spinal fluid can flow a little bit easier when you’re laying down,” he said.

He was just over 50 years old.

“It’s not a good feeling when your brain starts messing with you,” McMahon said. “You don’t know what to do.”

Several former NFL players have died by suicide. Junior Seau died in 2012. Aaron Hernandez was found dead in his prison cell in 2017.

McMahon played with Dave Duerson, who left a note before his death by suicide in 2011 asking to have his brain examined.

“The brain was just telling them to do some bad things and I couldn’t believe it,” McMahon said. “But after I started having those same thoughts myself … had I had a gun, I wouldn’t be here (today) because my head hurts so bad that the only way to get rid of it is just be done. And  thank God that didn’t happen.”

He blames the NFL.

“They need to take care of the guys that put their lives on the line for this sport,” he said. “If you remember, the NFL was not doing well as far as the public and that (1985) Bears team actually brought a lot of excitement back to football.

McMahon said he played through at least five concussions. He said the medical emphasis at the time was on joints, not brains. He was one of the first to sue the NFL, in 2011, for failing to properly treat players for concussions and trying to conceal for decades any links between football and brain injuries.

More players filed suit, and in 2017 the NFL concussion settlement was finalized. It was meant to compensate former players who suffer from cognitive issues related to their time in the NFL.

“There’s no doubt (the NFL knew) what’s going on,” McMahon said. “They’re very powerful people and they can, they can make stuff go away.”

He said he’s trying to help other players navigate this confusing disease.

“This is my life too,” he said. “I want to find out what the hell is going on with me, and at least now I know, but a lot of these guys do not, and that’s still the biggest problem.”