09/11/2008 at 09:40 AM EDT
After revealing in July that she has hepatitis C
, Grammy winner Natalie Cole is opening up about the agonizing treatment she's getting for the liver disease.
"I give myself a weekly injection of chemotherapy in my thigh," Cole, 58, tells PEOPLE in its new issue. "When I started in May, I thought I was dying. I couldn't get out of bed for three weeks – literally. I was nauseous every day. I lost 15 pounds from not eating."
Doctors diagnosed the disease in April after a routine blood test. They say Cole likely contracted hepatitis C
– which remained dormant in her body for 25 years – from sharing dirty needles when she was a heroin addict
in the early '80s. "My life crumbled before my eyes," she tells PEOPLE.
"I've learned a lot of lessons," Cole adds. "Yes, I could have handled some things better. But they've also made me who I am today."
Although she is still undergoing chemo until the end of the year to improve her chances of being cured, Cole's doctor confirms she is already virus negative after more than three months of treatments.
Cole says her "the show must go on" philosophy has helped her cope with her health issues. Her new album of pop standards, Still Unforgettable
was released on Sept. 9, and she is planning an October tour
The new album, a sequel to her 1991 Grammy-winning CD Unforgettable ... With Love
, features a new duet with her late father Nat King Cole called "Walkin' My Baby Back Home."
"I've worked so hard on this record – it's the first one I produced – I couldn't see pushing it back [while I went through my treatment]," says Cole. "So we're going to barrel through and kick butt. And I if I have to, I'll kick butt sitting down."
Other celebrities who have publicly battled the disease – which affects about 200 million people worldwide, including 4 million in the U.S. – include Pamela Anderson, David Crosby and Naomi Judd.
Cole says Judd has been a source of support since her diagnosis. "Sometimes she leaves me little messages. You'd be surprised how that support goes a long way," says Cole.
For more of Cole's story in her own words, pick up the new issue of PEOPLE, on newsstands Friday.