Brooke Mueller -- I AM at Cirque Lodge ... and I'm SERIOUS About Sobriety

Unlike Demi Moore ... Brooke Mueller IS being treated at the famous Cirque Lodge rehab facility in Utah ... and we're told she's making MAJOR progress in her quest to shake her demons.

Sources close to Brooke tell TMZ ... Brooke checked in to C.L. on December 12. We published a photo of Brooke boarding a plane bound for the Utah facility.

Sources say Brooke is confident this stint will be successful because she made the decision to check into Cirque on her own ... because she wants to get clean ... and not because of pressure from other people.

We're told Brooke's current stint is the longest she's ever stayed in rehab ... and she plans to continue the program for another 30-45 days.

Brooke spent two months in an in-patient program at the Lodge ... but we're told she recently began renting a home in the area and has been moved to an out-patient program so she can begin her transition back to normal life.