The TV presenter, 53, is due to front Channel 4 charity fundraiser Stand Up To Cancer on Friday (15 October) and admitted she had found it tough to hear that Harding had regrets over not seeking medical help about a pain she was suffering from earlier.
Former Girls Aloud singer Harding died in September aged 39 after being diagnosed with breast cancer a year earlier.
McCall told The Sun: “She’d given herself a hard time for not going to sort it out earlier. That, for me, is always the hardest thing."
The Long Lost Family presenter told how her own sister also "gave herself a hard time" after being diagnosed with terminal lung cancer, which she passed away from in 2012.
She continued: “There’s nothing like the look on someone’s face when they realise that what they’re being told is there’s no treatment. This is palliative care from now on.
“And the idea that you’d ever blame yourself for that is too much to handle. So we need to help people to find ways of detecting changes in their bodies, but never shaming anybody, ever, ever, ever, and least of all shaming yourself, ever. If you get sick, it’s not your fault.”
McCall first met Harding when she was hosting Pop Stars: The Rivals, the 2002 show which made Harding a star when it cast her in the group Girls Aloud.
The pair also worked together again in 2016 when Harding took part in winter sports challenge show The Jump, which McCall was hosting.
McCall, who also lost her mum to cancer, regularly hosts the Stand Up To Cancer fundraiser which includes comedy sketches, entertainment shows and stories from those who have been diagnosed with cancer.
This year, McCall will be joined again by Maya Jama, Adam Hills and Alan Carr.
Watch: Sarah Harding's consultant says she 'fought incredibly hard'