There's a reason Britney Spears' conservatorship has lasted 4 1/2 years with no end in sight -- it's because she has a serious medical issue that prevents her from making consistently sound decisions ... TMZ has learned.

Documents were filed Tuesday in the conservatorship case, urgently asking the judge to seal certain records.  In the papers, lawyers for the conservators called the records "highly sensitive," adding "irreparable harm and immediate danger" would befall Britney if the documents weren't sealed.

Multiple sources with firsthand knowledge tell TMZ ... the records are medical documents outlining Britney's condition and treatment. 

We're told the singer has a disorder affecting her personality that can affect her state of mind.  We're told she is doing "extremely well" but needs the safety net of a conservatorship.

Sources tell us there is no plan to end the conservatorship, and the judge is completely down with it.

FYI ... there are 2 components of the conservatorship -- the personal and the business.  The personal conservatorship allows Jamie Spears and Jason Trawick to make life decisions for her.  The business conservatorship involves managing Britney's career and fortune.  As for the business side, we're told it will probably go on indefinitely because it has been incredibly successful.