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Kevin Federline has been making the media rounds to publicize his new tell-all book about life with ex-wife Britney Spears ... and she's already had enough of his BS!

KFed joined us on "TMZ Live" Wednesday and we asked him if he thought it was a mistake to end the conservatorship ... among a slew of other Brit questions. Play video content

Kevin says everyone in the family -- including Britney's two sons Sean Preston and Jayden James -- are concerned for her well-being and he says his new memoir, " You Thought You Knew ," is a cry for help to gain support for Britney and their kids.

A few hours later ... Brit got on social media to fire back at Kevin, saying she's over his "constant gaslighting" -- and anyone who knows her, knows he's just trying t

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