In the book “The Spice Girls Revisited” by David Sinclair it was confirmed by the co-writers/producers of the song Matthew Rowe & Richard Stannard that Geri Halliwell was the main songwriter of “Viva Forever”, the last single by the Spice Girls as 5 piece. The song was written/recorded in 1997.
As for the meaning of the song, it is said that it’s about a romance that she kept a secret [Feelings unfold, they will never be sold and the secret’s safe with me] and it mostly reflects on the holidays, adventures and memories they made together.
The funny thing is: Geri never had a long term boyfriend during the Spice Girls or had the time to date anyone outside fame. Plus, most of the holidays/adventures she went was with Melanie B [Fiji, Portugal, Spain, Sri Lanka, Antigua and Barbudas, Maui, the infamous Mojave desert incident]. At the end, it pretty much explains why the song never mentions a “he”.
Bottom line: The song is about her relationship with Mel B.
Note: Mel B cried while performing this song twice in Oslo, Norway during the ‘98 Tour. The 28th of May (the day after Geri left) and the 29th (on her 23rd birthday).