EXCLUSIVE

Mama June and Sugar Bear Thompson are done -- separated -- because she's convinced he's been cheating on her ... sources tell TMZ.

We're told Mama June has repeatedly caught Sugar Bear trolling on online dating sites. One of them is plentyoffish.com, in which Sugar Bear -- with the handle Georgiafighter31054 -- says "i love to hunt fish and ride 4wheelers and have a good time. i luve muddy Boggs and love to ride in the mudd."

Mama June has stopped wearing her wedding ring (technically the couple was never married, but they did have a commitment ceremony).

Our sources say Mama June plans on taking the kids and moving out -- once the latest round of filming "Here Comes Boo Boo" is done ... so she can be closer to her relatives. She hasn't fully decided on the move.

We're told the producers of the show are scrambling to try and figure out what's next.