"Marrying Kevin was the last thing I was thinking about doing," Spears tells People magazine in its July 12 issue. "But then I said, 'You know what? This is my life and I don't care what people think. I'm going to get married. I'm in love with him.'"
Spears, 22, and Federline, 26, began dating a few months ago, after her Las Vegas wedding to childhood friend Jason Alexander (search) in January. That marriage was annulled 55 hours later.
Of the quickie Vegas ceremony, "That thing was a total ugh," she says. "I was not in love at all."
This is different, says Spears, who's wearing a five-carat diamond set atop two slender platinum, diamond-encrusted bands on her left finger.
Federline popped the question on an airplane while flying with Spears from Ireland to New York after the European leg of her tour. "I'd known for a while that she's the one," he says.
"I kissed a bunch of frogs and finally found my prince," says Spears. "I feel like I've found my happily ever after."
Federline, who performed as a backup dancer for Justin Timberlake (search), Spears' former boyfriend, previously was involved with Shar Jackson, star of TV's "Moesha." They have a 2-year-old daughter and are expecting another baby.
"I plan to meet his daughter," Spears says. "I love little ones. I think the situation is good."
The singer says she wants children of her own someday. "I'm not pregnant. (But) I definitely want to have some kids. I see myself with four or five."
She adds: "We're starting with a dog. I just got a Maltese named Lacy."