The ex-NBA player spilled his guts about his coupling with Kim K in The Players' Tribune ... saying, "Look, I should have known what I was getting into."

Kris and Kim began dating in 2010 and got engaged in May 2011. They tied the knot August 20, 2011 ... and Kim filed for divorce on Oct. 31.

"I was definitely naive about how much my life was going to change," Kris says ... "But the one thing that really bothers me is whenever people say that my marriage was fake."

"There’s definitely a lot about that world that is not entirely real. But our actual relationship was 100% real."

Kris says "it sucked" when he realized the marriage wasn't working -- "It’s never easy to go through the embarrassment of something like that — with your friends, with your family."

"But when it plays out so publicly, in front of the world, it’s a whole other level. It was brutal."

Humphries -- now 34 years old and out of the NBA -- says he didn't know how to handle the negative attention stemming from his breakup ... admitting, "There was about a year where I was in a dark place."

Kris says he didn't want to leave his house and didn't even want to be known as Kris Humphries. Instead, Kris says he pretended to be Blake Griffin when he was out in public.

He also reflects about how much it sucked to be booed and taunted in the wake of the split -- wondering if fans were after him because they thought he was a wannabe reality star ... or because they thought he disrespected the game of basketball.

Humphries says he's not asking for pity -- and never really wanted to be famous -- claiming he's now trying to stay "under the radar" as he works on the next phase of his life.

So, what is Kris planning to do next? He's already opened up several restaurants -- 10 Five Guys burger joints and with plans to open 7 CRISP & GREEN eateries.