The 37-year-old singer's Girls Aloud bandmate died of cancer in September 2021 aged just 39 but the 'Beat of My Drum' hitmaker admitted she still pretends to herself that her pal is away on holiday.
She admitted on 'Good Morning Britain': "I think that everybody deals with grief in different ways.
"For me it's still not really that real. A lot of the time I'm like , 'She's in Thailand with her friends, or in Ibiza'' - but then I realise that it's not real."
Although Nicola is proud to be pushing forward the BCAN-RAY (Breast Cancer Risk Assessment in Young Women) project - which was set up to find new ways to spot signs of the disease - in memory of her friend, she finds it difficult to talk about her.
Asked how she felt, she said: "It's unbelievably conflicting, and I have to somewhat emotionally switch off to get through the chat.
"From something so heartbreaking we are here to speak about something so hopeful. You have to focus on pushing forward or you fall apart. It's harrowing.
"Sarah and doctor Sasha became very close, and he told her about this breast cancer project. He became someone she could lean on. She said , 'I wanna help you, I wanna try and take this on.'
"Her wish was to take part in the fundraising that we did last year...
"For her, for her mum at home, her family and friends, she'd already created a legacy in her performances, and her singing and songs.
"But then to add this scenario of such magnitude to her legacy - and we can take that baton for her - was so important for her, and it's so important to us."
The former 'Masked Singer' winner went on to pay tribute to her "vivacious" and "unpredictable" friend.
She said: "She was present in every moment. She brought her self to every moment. She never tried to be anyone else.
"You got Sarah in all her glory. So authentic. The vivaciousness. She was sparky and off-the-cuff. She was the unpredictability in our group. She's just so missed."
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