NASHVILLE, Tenn. – Carrie Underwood, nervous?

You bet.

The former “American Idol” winner sold 6 million copies of her debut album “Some Hearts,” a phenomenal number for any artist let alone a new one. She won two Grammy awards, scored three No. 1 country hits and shot to superstardom almost overnight.

On the eve of her follow-up, “Carnival Ride,” she’s entitled to a few butterflies – or a whole swarm of them if she wants.

“It’s so anticipated. Not just by the public, but by us, too,” she says of the disc, which hits stores Tuesday. “The whole ‘Can we top the first one?’ mind-set sets in.'”

But Underwood says she won’t make the mistake of measuring success strictly by the numbers, especially because her debut set the bar improbably high.

“Even if it doesn’t sell as many, I feel like we’ve made a better album, which is what you want to do. You want to keep getting better and have better songs and keep sounding better and moving forward. So even if we don’t reach the numbers, I’m definitely still very pleased with it. I don’t think it will be a letdown at all.”

The first single, “So Small,” is No. 5 on Billboard’s Hot Country Songs chart and rising. Like her breakthrough hit “Jesus, Take the Wheel,” the lyrics and the music are uplifting, proclaiming “when you figure out love is all that matters after all, it sure makes everything else seem so small.”

The track is one of four that Underwood co-wrote, a step up from her one co-write on the debut. In fact, she had a larger hand in the whole project because there was much more time compared to the tight schedule after her “American Idol” victory in 2005.

“I was in the studio whether we were recording or not. If Mark (producer Mark Bright) was doing something I’d come by and listen to the background vocals that were being put down, and if I found something I didn’t like maybe I’d tell the background vocalists that I think it would sound better if we did it like this,” she says. “Mark was super open because it’s my voice and my album, and in the end I’m the one who should be most pleased with it.”

Sony BMG Nashville chairman Joe Galante says Underwood has grown since her “Idol” whirlwind, and it shows on the new record.

“I don’t think anything prepares you for what happened to her in that two-year period, especially with the speed at which it happened. No matter how grounded you are or how many people you have telling you this is what to expect, until you go through it you don’t know,” Galante says.

“She’s going to be around for a long time. She’s got a career. I think that will be the biggest measure of her success,” Galante adds. “She’s on her way to having a long-term career in this business where people are gone the next year. She’s not one of those people.”

A petite blonde of 24, Underwood was friendly and talkative during a recent interview at her management office – in sharp contrast to a nervous one she gave AP just before her first album. But she says she’s still shy and reserved in some situations.

“I look back at the ‘Idol’ tapes and I look horrified. I’m really amazed people still voted for me. I think I’m better at it now. But in social situations I’m still really shy. I’m not a great people person. I’m not good at initiating conversations or carrying conversations or anything like that.

“But I think now at least I can kind of turn it on when I get on stage. I do better and feel more comfortable on stage, but it’s taken a while,” she says.

Born and raised in Checotah, Okla., Underwood, the youngest of three sisters, began singing in church and later in school musicals and talent shows. But without “Idol,” she says she never would have come to Nashville.

“No way. I have to have a plan and have everything laid out in front of me. Packing up and hoping never would have been enough for me,” she says. “I’m too rational. I’m not enough of a dreamer for that.”

There were drawbacks to her fast climb. She’s the first to admit she wasn’t ready for all the attention. She likes her privacy, and it’s become harder to find. Most recently, her relationship with Dallas Cowboys quarterback Tony Romo has been a source of speculation (for the record, Underwood says, they’re just “really good friends”).

Last year, she was thrust into controversy at the Country Music Association Awards when Faith Hill seemed to storm off in anger after Underwood was announced the winner of the female vocalist award.

Hill, who was also up for the award, said she was only goofing around and called Underwood that evening to apologize. Last month, Hill said she was so upset by the incident that she considered quitting the music business.

“She and Tim (Tim McGraw, Hill’s husband) have always been so nice to me. I wouldn’t have any reason to think it would be anything other than her being goofy backstage,” Underwood says now. “I even think I told her ‘I’m sorry for what’s about to happen.’ She is one of country music’s darlings.”

Underwood may understand as well as anyone. Like Hill she’s been held up as America’s sweetheart, and like Hill she’s taken some shots, perhaps because of it.

“I love what I do and I wouldn’t trade it for anything, but sometimes people make up stuff and it hurts my feelings on a media level and on a personal level when someone tells me, ‘I heard you made some little girl cry’ or something like that.

“I consider myself to be a no-ripples-in-the-water type of person and I don’t want to make anybody mad and I try to make everybody happy. It’s something I’ve had to deal with, realizing that I can’t,” she says. “I’m still working on that I think.”