The Hollywood superstar lays bare his 21-year marriage to Jada in a candid discussion for an upcoming episode of her hit "Red Table Talk" series, and in a preview of the chat, he admits there was a time in their union when they really struggled.
"There was a period where mummy woke up and cried 45 days straight," Will confesses to their 17-year-old daughter Willow Smith, who co-hosts the Facebook Watch show alongside Jada and her mother, Adrienne Banfield-Jones.
"I started keeping a diary," the "I Am Legend" actor shares, prompting Jada to quip, "You missed some days."
"I think that's the worst I ever felt in our marriage," he continues. "I was failing miserably."
Will didn't reveal when the low point occurred in the teaser, but the new episode of "Red Table Talk" debuts on October 22.
The intimate discussion marks the first time he and Jada have really delved into the complexities of their marriage, which many fans consider to be one of the strongest in Hollywood.
However, they have both insisted maintaining a strong bond takes work.
In an interview on the Rap Radar podcast back in July, Will said, "We don't even say we're married anymore. We refer to ourselves as life partners, where you get into that space where you realise you are literally with somebody for the rest of your life."
"There's no deal breakers. There's nothing she could do - ever. Nothing that would break our relationship. She has my support 'til death and it feels so good to get to that space."
His comments echoed similar statements Jada made on radio show Sway in the Morning weeks earlier, when she insisted that her husband can rely on her "for the rest of his life, period."
"Here's the thing about Will and I - (we) are family. That (divorce) is never going down. It's just not. Ever," the 47 year old declared. "We are family. Take out all that whole marriage/relationship c**p, at the end of the day, Will and I are family. I am going to hold him down...."
The couple, which also has 20-year-old son Jaden Smith, wed in 1997.