Sheryl Crow recently turned 40.

"So what!" she declares. "Forty is the new 30!"

Anyway, she adds, "It doesn't mean a damn thing."

Yet Crow would be the first to admit she has struggled in the last few years with the same questions that nag many high-achieving, single women of her age, even in these postliberation days. Namely, how to find time for a marriage and/or children in the midst of a brilliant career.

And Crow's career has been more brilliant than most. Her first three studio albums each sold in the multimillions, and she remains a major concert draw around the world. Yet, she says, "You lose huge chunks of your life when you go on the road or into the studio. It's a very solitary, difficult thing."

So isolating and strenuous is it, in fact, that, two years ago, when she first tried to cut her latest album, "C'mon, C'mon," Crow faced serious psychological problems.

"I was stuck in my personal life and in my music. I wasn't able to finish anything," she says of her album, which finally hits stores tomorrow. "I'd been in this work mode for 10 years. I needed to reestablish the balance of home and work. I was in direct conflict with my feelings."

So she wound up taking seven months off to "figure out how to make [the music] new again."

In the meantime, Crow became nervous about losing career momentum, and about how she would fit into a current pop scene filled with teen queens half her age and hip-hop stars from a very different background. "It had my head spinning. People were encouraging me to put [hip-hop] beats in my record. But it felt calculated."

"It's not unusual for an artist who's been around for more than a minute to be concerned about where they fit in the current pop landscape," comments Billboard's chart maven, Geoff Mayfield. "But she has a solid track record and a core audience. So her prospects are quite healthy."

In the end, Crow didn't make a trendy record. Instead, she went back to the sound that first inspired her — the summery, Southern California rock of '70s bands like Fleetwood Mac. The result is Crow's most upbeat album to date, filled with happy hooks and scratchy little rock riffs.

Several songs find Crow in an appreciative frame of mind. But they came from hard times. "In making this record, I was missing a life that's more simple," she explains. "The amount of everything we're all exposed to can cause depression. We have e-mail, pagers and cell phones. We're traveling at such a high rate of speed and we fill our days with minutiae."

Crow has been just as concerned about the flimsiness of current pop stars filling the airwaves. The new song "One in a Million" is her rank on interchangeable artists, though she's loath to name names. She plays it just as close to the vest in the new "Over You," which deals with the consequences of breaking up with a famous person.

Enquiring minds want to know: Could she be referring to any of her famous exes, like, say, actor Owen Wilson? "I'd like to leave that open for debate," she says with a laugh.

Crow seems to be a magnet for romantic rumors. She was erroneously linked to Kid Rock "just because someone took our picture together at a Knicks game." And she got yoked with Matt Lauer because, she says, "We were on the same Letterman show."

Dubious reports also flew that she was replacing Christine McVie in Fleetwood Mac. "I don't know why that came out," she says. "Part of it is just because I toured with Stevie Nicks and worked on her record."

In fact, Nicks recently told The News that Crow "is more like someone of our [boomer] generation than of her own. Which is to her credit. She should have come up in the '70s."

Small wonder Crow and Nicks have become good friends. Besides, they're both highly successful rock 'n' roll women without husbands or children. While Nicks has gone on record as saying she didn't think she could "have it all," Crow believes she can.

Yet she acknowledges that it's easier for men. "We don't have a lot of history of male rock stars being the kid raisers."

Though Crow says she thinks she will marry someday, she isn't surprised by her single status. "I've had great relationships but they're not ones I could have maintained. Creative people are drawn to creative people. And I don't know if [those] relationships are conducive to lasting."

Of course, sustaining a career has its problems and its compromises, too. For promotional purposes, Crow lent her latest single, "Soak Up the Sun" to an American Express ad. That's not uncommon these days, but this one makes an uncomfortable fit, since the song's lyrics urge listeners to appreciate what they have, while the ad encourages shopping sprees. Crow admits, "It's a touchy subject. [But] I have a good relationship with American Express. They financed my Central Park concert and gave money to pediatric AIDS."

Regardless, the ad has brought attention to Crow in a competitive market. So has a major cheesecake shot of her that appeared on a recent cover of Stuff Magazine. She says the provocative shot wasn't threatening to her image because she never sold herself on sex.

Yet the cover does wind up raising the issue of age again, only this time with pride rather than indifference. "I'm 40," Crow says with joy. "At this age, it's fun that I can do something like that at all."