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Celebrity autobiographies face formidable challenges. Not least, there is a fine line between satisfying readers’ obviously insatiable hunger for dirt — a menage a trois will do nicely, thank you — and drifting into New Age navel-gazing.

From the beginning of her new autobiography, “My Life So Far” (Random House, 599 pages, $26.95), Jane Fonda struggles to navigate that course between preachiness and passion, between true insight and therapy-speak:

Fonda’s experiences are exceptional fodder for a life story, from being the privileged child of a Hollywood icon, to a sci-fi sex kitten, to an Oscar-winning actress, to an impassioned (if sometimes wildly misguided) activist, to a feminist firebrand (who defined herself entirely by the men in her life), to a fitness tycoon (who spent decades bingeing and purging), to a grandmother and philanthropist.

A deep sadness permeates virtually every page of “My Life So Far.” Readers are reminded over and over that her father, Henry, was a cold, cold man; that young Jane had serious issues with her body and her self-image; that some men cheat. Fonda reportedly wrote this book herself, casually dropping in quotes and insights from Rainer Maria Rilke to Bonnie Raitt, from the Bible to “The Velveteen Rabbit.” The most stirring, well-written portions of the book are about making movies.

The least-effective portions of the book cover the issues that Fonda appears to care most about: preventing teenage pregnancy, saving the environment, empowering women. Although Fonda writes about recognizing the shrillness of some of her early activism (“Watching some taped interviews years later with my son … I wanted to shout, `Will someone please tell her to shut up?'”), the humorless approach makes the chapters about her causes far too skimmable.

Fonda struggles valiantly — and unsuccessfully — to explain the tortured, complicated relationship she had with her father and with her three husbands: French director Roger Vadim, political activist Tom Hayden and irrepressible media mogul Ted Turner. But it’s when she’s not trying so hard that her life story becomes truly compelling — when she’s drawing a magnificent, richly nuanced portrait of Katharine Hepburn, or recounting a tour of Appalachia with Dolly Parton as a guide, or remembering a young Marilyn Monroe as she cowered in the back of an acting class.