Beckham added that she is continually "reinventing" herself and "ignoring the naysayers."
"You always look beyond the conventional wisdom to pave your own path," she wrote. "First, you found this passion in fashion, and most recently, beauty. What comes next? I'm dying to know."
The recent letter is a follow-up to a letter she wrote to her 18-year-old self back in 2017, where she also reflected on her time with the group.
"Together you will make it OK to look different. And, as the Spice Girls, you will sell 75 million records," she wrote to her younger self in 2017. "You cannot possibly imagine your future life right now. You will travel on private planes, visit incredible countries, stay in fantastic hotels."
"You are lucky to be there," she added. "If you don't join the Spice Girls, you might always be that insecure person in that little shell, and you will never become who you truly are."
Beckham remained with the Spice Girls until the end of their 2012 reunion. She didn't join former bandmates Geri Halliwell, Melanie Chisholm, Melanie Brown and Emma Bunton when they decided to get back together in 2018.