TMZ Sports talked to the 38-year-old former UFC champion turned boxer/PFL fighter just days after he dispatched Renan Ferreira via a first-round knockout in Saudi Arabia ... and then broke down in tears after the bout while talking about the devastating loss of his little boy, Kobe.

"Oh, yes [I didn't know if I'd fight again]. There was a point that you didn't know anything. Everything seemed so fragile," Francis told us, adding his mind was racing a mile a minute during that horrific time.

"You didn't know if it's like some sort of genetic pandemic, genetic disease. You didn't know if it was like a curse. If it was just bad luck that's going to keep happening. There was a point that, you know, like a lot of stuff happened in your head, stuff that you don't usually think about."

Ngannou made the fight look easy ... but mentally and emotionally, says it was "by far the toughest" of his career.

Ultimately, when deciding when to step back in the cage, Francis came to the conclusion that the pain would never go away ... so there was nothing to wait out.

"I didn't feel like there would be a time that it's going to go away, where it's going to be okay. It's one of those things that you just realize at the beginning that you better learn how to roll with it because it will always be there."