The singer, known as Ginger Spice, left the four other girls in the middle of their 'Spiceworld Tour' to pursue a solo career in 1998.
Although all five girls - Victoria Beckham, Mel B, Mel C, Emma Bunton and Geri - have reformed, Geri's bandmates are still finding it hard to forgive and forget.
Mel B said: "It was horrid when Geri left, horrid."
Victoria added: "When Geri left, it was really difficult for all of us. But we had the American leg of the tour to fulfil, and nothing was going to stop us from doing that. But we were devastated. The Spice Girls is about five girls."
Geri, 35, has promised to make it up to the girls on their reunion tour, which kicks off on December 2, in Vancouver, Canada.
She said: "I'm sorry. I'm making amends this time. I made sure we got America in the tour schedule."
The girls also confessed they were nervous when they made their comeback debut at the Victoria's Secret Fashion Show last week.
Mel B said: "I was actually thinking, 'Thank God. They're happy to see us all.' They loved it."
Mel C added: "When that roar came, it was hard to hold yourself together."
Emma Bunton agreed: "It was a bit emotional."
The Spice Girls' reunion tour begins on December 2 in Vancouver.
TAGS: Music Spice Girlsgeri Halliwell