The 39 year old former singer shared her diagnosis in an emotional Instagram post last year.
And now, she has opened up on on her health battle in her new book Hear Me Out, published by The Times, and is urging people with health concerns to see a GP.
Sarah admitted she avoided seeing her doctor because of coronavirus and revealed how she thought she had a cyst before her diagnosis.
Get exclusive celebrity stories and fabulous photoshoots straight to your inbox with OK!’s daily newsletter. You can sign up at the top of the page.
She wrote: “At first I thought it was just a cyst. The trouble was the pain was getting worse. It got so bad that I couldn’t sleep in a bed. Eventually my skin started to bruise. By now I was terrified.
“One day I woke up realising I’d been in denial. Yes there was a pandemic but it was almost as if I’d been using that as an excuse not to face up to the fact that something was very wrong.”
Sarah did open up about her cancer diagnosis with her fans in the candid Instagram post, but didn’t reveal how she was rushed to intensive care before her chemotherapy began as she was diagnosed with sepsis.
She wrote: “With both my lungs and kidneys failing, doctors decided to put me into an induced coma.
“Even once I was off the ventilator I couldn’t speak properly. All I could do was make noises like a chimpanzee trying to communicate.”
Sarah, who recently broke her silence to give an update on her cancer battle, then went on to start her chemotherapy treatment and have a mastectomy.
Reflecting on the mastectomy procedure, which is an operation to remove a breast, she added: “I remember the surgeons saying they would make a drawing of what parts of my breast needed to be taken away and it was virtually all of it.
“I’m so glad that Mum was there waiting for me because when I woke up I screamed the place down. Now there’s just a bunch of stitches where my breast used to be.
“I can’t look at myself in the mirror anymore. I don’t recognise myself.”
In her touching book, Sarah has also revealed that she was told by doctors that Christmas 2020 would be her last, but she is remaining positive.
The singer wrote: “In December my doctor told me that the upcoming Christmas would probably be my last.”
She added: “I don’t know how many months I have left. Maybe I’ll surprise everyone.
“I’m trying to live and enjoy every second of my life, no matter how long it may be.”