Harding died in September at the age of 39 after battling breast cancer.
Opening up about the loss, Walsh told Psychologies magazine that she struggles with it every day.
Read more: Sarah Harding's 12 greatest moments
“I can’t lie, it absolutely knocked us all for six and the grieving is a daily challenge,” said the mum-of-three.
“Until you’re actually experiencing something like this with somebody who was close to you, it’s impossible to imagine what it’s going to feel like.
“And I honestly still can’t work out how you ever truly accept it.”
Walsh, 40, said that for her, dealing with her grief means “checking in with everyone, especially Sarah’s mum” on a regular basis.
“It’s going to take time, and it’s very, very tough,” she said.
Harding revealed in August 2020 that she had been diagnosed with breast cancer and that it had spread.
Her mother Marie announced on 5 September this year that she had passed away, saying her “beautiful” daughter was “a bright shining star”.
Countless stars paid tribute to the singer, with fellow Girls Aloud star Cheryl saying that she was “stunning, unique, crazy, quirky, kind and soft-hearted”.
Bandmate Nicola Roberts described Harding as “Electric girl” in a moving Instagram post, adding: “You made us. You gave it everything and still with a smile.”
Watch: Sarah Harding 'gave herself a hard time' for not discovering cancer sooner