knowing nothing about the laws that govern conservatorship, it's a bit hard to believe that a Court would allow one adult to take complete control over another adult, without a fairly compelling basis for making such an order. I would also be shocked if the system didn't go out of it's way to make sure a person like her received adequate legal counsel/representation throughout the whole process.
Britney is probably a nice and decent person who sadly lacks the mental tools required to take care of her rather complex life. For medical privacy reasons, there is probably a lot of hairy details the public doesn't know about. Also at any point Britney could've searched google for "how to end a conservatorship" and seen a thousand articles that say how to do it. Seems like her past attorney has told her that it is unlikely, for whatever reasons she wanted to provide, that she could get out of the conservatorship, and she interpreted that to mean "you can't get out of the conservatorship." Attorneys like billable hours so they are not going to advise otherwise unless it's pretty clear the motion will fail.