The "Bram Stoker's Dracula" star was married to the "Sherlock Holmes" actor between 1997 and 2003 and the pair shares children Rafferty, 24, Iris, 20 and Rudy, 19, but she revealed having the kids so close together took a toll on her mental health.
Speaking on "The Moments That Made Me" podcast with Roxie Nafousi on Tuesday (14Sep21), Sadie said, "Because I had had two children very close together and my life was going a hundred miles an hour, I got very serious post-natal depression. And that is when my whole world came caving in and it was scary for quite a few years on a daily basis."
The actress and fashion designer explained the experience taught her not to "undermine" the importance of her mental health and helped her to understand the "bigger picture."
"I fought really hard and what it taught me was one, don't undermine ever having your sanity, your confidence or your self-esteem and it showed me to have compassion and understand the bigger picture," she said.
Frost, who also shares son Finlay with Spandau Ballet star Gary Kemp, believes her battle with post-natal depression "moulded" her into the person she is now.
"It made me more determined to be a strong mother, more career-orientated and help other people," she added. "Take out, but pack in, and make other people's lives better and anyone who is in pain, try and help as much as possible. For me, it has moulded me into the person I am today."
Despite the progress she's made, she shared she will "never completely mend."
"There'll always be that post-traumatic (stress) or something that can trigger the fact that you're on the edge. You're a slightly broken, damaged person and you'll never completely mend it. I take... my mental health seriously."