After Tom Brady's son was born, "He opened my heart in ways I didn't even know my heart could expand," Gisele Bündchen tells PEOPLE in this week's issue
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FINAL of Gisele Bundchen.
Credit: Nino Muñoz

Gisele Bündchen found her romantic match in 2006, when she was introduced to now-husband Tom Brady by a mutual friend.

But just two months into their relationship in 2007, they discovered that his ex-girlfriend, Bridget Moynahan, was pregnant with his first child: son John “Jack” Edward Thomas, now 11.

“It wasn’t the ideal situation for anyone — there’s no manual to prepare you for that,” Bündchen tells PEOPLE in a revealing new interview for this week’s issue.

“But I’m so grateful for [Bridget],” she says. “I know this was hard, but I couldn’t imagine my life without [Jack]. I call him my bonus child.”

After Jack was born, “He opened my heart in ways I didn’t even know my heart could expand,” says the supermodel and Lessons: My Path to a Meaningful Life author, 38. “I fell in love with him.”

She says feeling such love for her husband’s son made her “want to start a family” with Brady, whom she married in 2009. And though they had previously discussed waiting 10 years to have children, they realized they “wanted Jack to have a brother or sister close in age.”

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Tom Brady with his kids Jack, Benjamin and Vivian
| Credit: Gisele Bundchen/Instagram
FINAL of Gisele Bundchen.
Gisele Bündchen
| Credit: Nino Muñoz

In her new memoir — Lessons: My Path to a Meaningful Life, written in the style of a journal — the famously private supermodel reveals that motherhood was a tough adjustment. (She and Brady, 41, share daughter Vivian Lake, 5½, and son Benjamin Rein, 8½.)

“Things can be looking perfect on the outside, but you have no idea what’s really going on,” Bündchen tells PEOPLE. “I felt like maybe it was time to share some of my vulnerabilities, and it made me realize, everything I’ve lived through, I would never change, because I think I am who I am because of those experiences.”

“When I became a mom, I kind of lost myself. It was like a part of me died,” she adds. “I’d been this very independent person. It was all about me. But now I had this little being, and I suddenly felt l couldn’t do other things and that was very hard for me. All I ever wanted was to be a mom, but when you’re actually experiencing that, it’s a shock.”

Gisele Bündchen’s memoir, Lessons: My Path to a Meaningful Life
| Credit: Peter Zambouros

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Bündchen also acknowledges that the stresses of parenting have sometimes taken a toll on her marriage. Taking on the majority of the childcare while Brady trains — often from 6 a.m. to 6 p.m. — “You’re overwhelmed and tired and then you’re not the nicest partner,” she admits.

The Brazilian model says she and her husband have had disagreements (his decision not to retire, for one) and weathered rough patches (like 2015’s Deflategate scandal, when Brady was suspended while NFL officials investigated allegations that he cheated by playing with under-inflated footballs).

“When someone you love is happy, it makes you happy, or if they’re sad it makes you sad,” she says. “You suffer with them and you have joy with them.”

All proceeds from Bündchen’s book will go to Projeto Agua Limpa.

For more about Gisele Bündchen’s new book and life at home as a mom and stepmom, pick up the latest issue of PEOPLE, on newsstands Friday.