Geri Halliwell-Horner on Advice She'd Give Herself During Early Spice Girls Days: 'Tranquilo!' (Exclusive)

Geri Halliwell-Horner has a message to all young adults out there: "You've got time."

In this week's issue of PEOPLE, on newsstands Friday, the Spice Girl, 51, reflects on "feeling the pressure to achieve, to get the job, get the partner" in her 20s and 30s, even amidst her '90s pop superstardom.

"I was a bit of a late developer, and watching other people get their personal lives sorted was quite challenging," Halliwell-Horner says. "I did not focus on personal life at all during my 20s and 30s, and there's all these boxes to tick off. I would say to my younger self: 'Tranquilo! Everyone's trajectory is different.' I had a baby naturally at 44!"

Halliwell-Horner indeed welcomed her son Monty, now 6, with her husband of eight years Christian Horner in 2017. (She also is mom to daughter Bluebell, 17, with ex Sacha Gervasi, and stepmom to Horner's daughter Olivia, 10.)

When Halliwell-Horner met Horner, the principal (i.e. manager) of Red Bull’s Formula One racing team, she was in her 40s and proudly able to say, "This is who I am."

"You know how when you're 12 you're in no-man's land? You're too big for the teacup ride, but you're not quite ready for the big roller coaster? I think that's the same with the 30s," she says. "You're not the ingénue anymore, and you're still finding your feet and what it means to be a fully fledged grown-up."

"When I hit 40, there wasn't much airbrushing going on, to put it politely," she continues. "I was my best geeky, silly self, and I think that helps to find a more compatible combination."

As to how she successfully blended her and her husband's families, Halliwell-Horner says there "isn't one particular answer."

"I don't think anyone is perfect," she says. "Whenever I think I've mastered being a good parent, the kids change again. I'm grateful that Chris and I are very supportive of each other. The other thing is time and allowing things to evolve organically. Love always finds a way."

Since becoming a mom, Halliwell-Horner — whose new young adult novel Rosie Frost and the Falcon Queen is out now — says her "perspectives and priorities" have totally changed.

"A child only has a childhood once," she says. "It's a lot of work being a parent, and at the same time, it's so rewarding. I feel so grateful that I've had the opportunity to be a mother. When I've watched my child achieve something, I'm so happy. I don't love them any more for their achievement — the love is always the same — but there's so much pride."

Like her mom, Halliwell-Horner's daughter Bluebell is a writer and budding poet. Just before Halliwell-Horner's PEOPLE interview, Bluebell accepted a prize for her poetry at school.

"She loves writing, and I just felt so happy for her," she says. "I felt a sense of pride. I'm the same with my husband. Maybe as you grow up, you become more like, 'Wow.'"

At home in England, Halliwell-Horner has her hands full with the kids, as well as their plethora of animals, including horses and dogs.

"I don't know how many animals I've got!" she says. "We bought another horse today, and we named him Hercules after my favorite animation. I learned to ride horses in L.A., and now Monty is giving it a go. I was shocked because as a parent, whenever I try and push or over-encourage my children, they're like, 'Meh.' He just suddenly went, 'I'm doing it.' I think he had a play-date, and he was trying to impress his little girlfriend."

She's also been busy promoting Rosie Frost and the Falcon Queen, which she will donate a portion of the proceeds from to the Barbara Bush Foundation's literacy work.

"I don't come from a privileged background," she says. "I got lucky and got sent to a really good school. I remember just sitting under the bottom of the tree in my garden in the summer holidays, and I'd just read. The Barbara Bush Foundation really tries to empower people at a grassroots level."

While fans have been excited about another potential Spice Girls reunion tour (they last hit the road together in 2018), Halliwell-Horner says there are no plans in the works for one...yet.

"We’ve got lots of things on our plate,” she says. “I think everything has its moment and its time. It’s always gorgeous to be with the other girls, so we’ll see.”

Though she's come a long way from her "Wannabe" days, Halliwell-Horner is "grateful" for all of her chapters.

"It's just different chapters, and you can just be the best version in that chapter," she says. "I'm enjoying each chapter for what they are. I feel very lucky and grateful for what I have and what I've experienced."

She's hoping she hasn't hit her peak yet, though.

"Some people peak in high school — they get the lead in the show, and that's their moment," she says. "I'm hoping I get to peak in my latter years."