"It was difficult, you know," Crow tells Vanity Fair magazine in its August issue. "I know he wanted to be there."
Crow and Armstrong announced their breakup in February. The 44-year-old rocker says the split was "devastating" for both of them.
"I do think about Lance every day," she says. "And I think about his kids every day."
When Crow learned she had cancer, she contacted the seven-time Tour de France champion, who was on a solo road trip from Lake Tahoe, Calif., to Oregon.
"When I got that news, I can't even describe it," says Armstrong, who is a survivor of testicular cancer. "It was the cruelest twist. ... I actually turned around to make the drive to L.A. We talked along the way and she said, `You know, I just don't think that's a great idea."'
According to Armstrong, Crow told him she felt "it would be hard on her in trying to manage the breakup and the diagnosis. And I totally understood."
The singer, who underwent surgery in late February, denies speculation that Armstrong, who has three children from a previous marriage, didn't want to start another family.
"It would be easy to say this is all about my wanting to get married and have kids but it was never that simple," she says. "It was much deeper than that."
Says the 34-year-old Armstrong of their split: "We were at different points in our lives."
Crow says her romantic relationships never seem to lead to the altar.
"I think that if I wanted to be married I would be married by now," she says. "And, for whatever reason, I haven't done it. I've picked people who've helped me to not make that happen."
Crow will begin a tour with John Mayer on Aug. 24 in Pittsburgh. A portion of the proceeds will be donated to The Breast Cancer Research Foundation.