The 38-year-old Girls Aloud singer announced last week that she is undergoing chemotherapy and that her cancer has advanced to other parts of her body.
Nolan, 61, is herself currently undergoing cancer treatment for the third time.
“If I can say anything to you, the support you’ll get from the public will be amazing – it was overwhelming for my sister Anne [who is also suffering from breast cancer] and me.
“People are genuinely lovely and want to wish you the best. Initially, I didn’t want people to pity me, but it was weight off my shoulders when it came out and I hope you’re feeling a sense of relief too.”
She went on: "This is my third time of being diagnosed with cancer now and each time is just as scary.
"But once you get your head around it – and you've got a supportive family, which I know you have – before you know it you'll be at the end of your chemo treatments.
"I started my chemo in June and thought 'I'm never going to get to September,' and now I've got two treatments left. And you get into a routine with it.
"Although you're feeling rubbish, remember it's treatment to make you better. As you've said, the doctors are amazing and give you so much support, but don't be frightened to ask questions."
Harding – who rose to fame after Girls Aloud won Popstars: The Rivals in 2002 – announced on social media that she was diagnosed earlier this year and is currently undergoing weekly chemotherapy treatment.
She said she was “doing my very best to keep positive”.
Nolan, who was first diagnosed with breast cancer in 2006, recently revealed her cancer had returned, at the same time that her sister Anne Nolan, 69, was also diagnosed with the disease.
Nolan lost her elder sister Bernie to breast cancer at the age of 52 in 2010.
The Nolan Sisters, including Coleen, Maureen and Denise, found fame as part of a girl group in 1974 with hits including I’m In The Mood For Dancing.