The 2012 Olympic gold medalist spelled it all out in a powerful video on her social media page Tuesday ... saying she's been in Biles' shoes before, and it's not fun at all.

"I've had a lot of experience with these mental blocks," Maroney said. "They are miserable. They are a gymnast's worst nightmare and they can end your career."

Maroney says during one of her experiences with the body yips ... she actually got seriously hurt, falling off a balance beam and ending up in a pool of tears with a concussion.

Maroney explained the blocks are caused by stress, depression and pressure ... but added they can come out of nowhere for even the best gymnasts too.

"Like, you can't even understand them," Maroney said. "Like, 'Why is this happening to me? I don't know why my body is doing something different.'"

The 25-year-old then went on to praise Biles for having the mental strength to pull out of Tokyo Olympic events over the issues.

"She was smart to protect her body and her mind," Maroney said. "Because it's more important. It just is."

Many other Olympians -- both current and past -- feel the same way as McKayla ... Michael Phelps also threw his support behind Biles on Tuesday.

"We’re human beings, nobody is perfect," the swimming legend said. "It's OK to not be OK. It's OK to go through ups and downs and emotional roller coasters."

"I felt like I was carrying, as Simone said, the weight of the world on my shoulders. It's a tough situation."

Added swimming star Katie Ledecky ... "I really hope that she continues to do what’s best for her and that the people around her, her coaches, her teammates, continue to support her."