DOLLY PARTON’s been letting it all hang out for decades, but the feisty country legend left a LOT of painful secrets out of her new memoir, say sources.

What the 66-year-old entertainer DOESN’T want fans to know is that she may not be able to continue per­forming because of crippling back pain brought on by her famous 42DD breasts. In her recently released book, “Dream More,” Dolly shares lessons from her legendary life. But she skips over some rough spots, like the messy details about her unusual marriage to reclusive husband Carl Dean as well as her heartbreak over never having a family of her own, says a close friend of the “9 to 5” singer.

“Dolly fears every tour will be her last,” said the friend. “She suffers from intense, crippling pain because of her legendary bust. She tries to stay away from overmedicating her­self and instead relies on meditation to ease the pain.

“But it’s gotten worse and worse as she gets older – and at 110 pounds, she’s just too top-heavy. It wreaks havoc on her back.”

Dolly is also suffering emotion­ally because of her unorthodox mar­riage to Carl, whom she wed in 1966. Even though they’ve been married for more than four decades, they live in separate homes and haven’t been seen together in public in years.

The two have had their share of affairs, and Dolly still grapples with the guilt she feels about having been unfaithful.

Dolly also strug­gles with the fact that she’s never had children, an issue another source says continues to intensify as the singer ages.

“Dolly always believed she’d have at least one baby, maybe two, with Carl, but her career and health prob­lems got in the way,” said the source. “Dolly kept on pushing the idea of having kids down the road as her career was skyrocket­ing, but when she got past 35, she and Carl decided it was get­ting to ‘now or never’ time.”

In 1984, when she was 38, Dolly suf­fered health problems which resulted in her undergoing a partial hys­terectomy. And that dashed all hopes of motherhood.

“Dolly has doted on her younger relatives,” the source pointed out. “She loves all of her nieces and neph­ews and always spoiled them rotten.

“But more and more, Dolly talks about what her life might have been like if she had kids – what they would be doing now, how many grandchil­dren she would have. I’ve seen her break down in tears on occasion and say, ‘You know, I would have been a great mom.’”

Professionally, Dolly is as popular – and as busy – as ever. She has sold over 100 million albums and last year performed 50 concerts, which grossed $34 million – her most successful tour yet.

“She’s gotten more reflective with age,” added the source. “While she definitely counts her blessings, I think she looks back at the heartaches as well as the great successes.”