I don’t necessarily think she’s bipolar. There are a number of other things that can induce mania and psychosis.
Britney has allegedly been heavily abusing drugs for a long time, predating the conservatorship. Uppers to keep her going, and downers so she can sleep. She has also allegedly had a number of overdoses, and a supply of narcan was/is kept at her house. At least one of those overdoses is alleged to have resulted in anoxic brain injury.
She’s been sued at least twice by former bodyguards. The first one accused her of sexual harassment, child abuse, and drug abuse. These went away with undisclosed financial settlements and NDAs.
Sam Lufti is another one that received a settlement to go away and shut up, after publicly making statements about what he had allegedly witnessed prior to the conservatorship. As odious as he was/is, they didn’t sue him for defamation. This alone doesn’t prove he was telling the truth of course, but what he’s said very much aligns with the things others have said.
Although it’s a drug associated almost exclusively with one demographic, methamphetamine is way more popular with celebrities and otherwise ‘respectable’ people than is realized. Cocaine doesn’t last very long, whereas meth can keep someone going full throttle for hours if not days.