Collin Gosselin’s out of the U.S. Marines, and he says it's because the Corps has an issue with something his mom Kate did back in 2016 ... sending him to an institution for troubled kids.
CG -- one of Kate and Jon Gosselin's famous brood of 8 -- says he was on the brink of graduation from the Marines when the service discovered his past stint in a mental health facility, and that led to his discharge.
He told Entertainment Tonight ... the Marines didn’t dig into the circumstances of why he was at PA's Fairmount Behavioral Health Institute -- it was just a straight-up exit, no ifs, ands or buts.
Collin's not hiding his resentment over Kate's decision to send him to Fairmount ... in the clip he explains why he thinks she only did it to cover up her own actions.
Kate's responded to his criticism ... saying her son had several psychiatric diagnoses back then, and she had no choice but to send him to the facility.
It's a rough turn for Collin -- just last year, his dad Jon was bragging about how great he was doing in Marine boot camp after enlisting at 18, then leaving to train at 19, following high school.
Jon clearly had his son's back ... especially since Collin moved in with him for 2 years following the whole institutionalization ordeal.
Collin's fighting the Marines' decision because nothing tops his dream of being a U.S. Marine ... but in the meantime, he’s planning on channeling his energy into a finance degree at Penn State.