Jennifer Aniston is more than fed up – she’s deeply bitter about ex-John Mayer‘s PR romp with a much younger woman.

The 31-year-old playboy singer is striking all the right chords with aspiring "actress" cocktail waitress Scheana Marie Jancan, who is only 23.

Jen, 40, claims she doesn’t care that Mayer has a new young girlfriend, but her catty comments tell a different story.

"Jen is hurt and brooding that John rebounded so quickly," an insider told The ENQUIRER.

"She went on a rant, telling friends that John should be with such a younger woman – because he’s so immature and juvenile."

The Friends beauty and the androgynous Mayer first hooked up in spring 2008.

"At first Jen was amused," said the insider. "But she was humiliated by his inability to hold back any private information."

During his much ballyhooed cruise to Mexico with fans- just weeks after he and Jen’s second and final breakup – John stunned her – and the world – by nearly coming out openly admitting to making out with celeb blogger/fame whore Perez Hilton, who frequently disses Jen on his self-absorbed website.

"Jen IS bitter," the insider divulged. 

– Emo Mayer’s already broken up with his new GF – in less than a month!