The German beauty parted ways with former "Dawson's Creek" actor Joshua in 2016 after 10 years together, and she began dating her current boyfriend, "The Walking Dead" star Norman Reedus, a few months later.
While Diane appeared in public to have quickly bounced back from the relationship breakdown, privately she struggled through a tough few months, which made her Best Actress win for "In the Fade" at the Cannes Film Festival in 2017 a bittersweet affair.
"I was in such a dark place," she recalled to iNews. "I had two really close family deaths that year, I was separating from my partner of 10 years."
"There was just a lot of darkness and personal loss. Then the role was very dark," she added of the crime drama, in which she played a woman who loses everything in a bomb attack.
Having her work celebrated at the annual French festival felt "superficial" to Diane, considering everything she had been through in the past year.
"There is something about grief and loss that lets you see how uncompassionate we are as a society, including myself," she reflected. "You watch TV and you become so numb to somebody else's misery."
However, Diane has since overcome the depressing period and has found happiness again with Norman, with whom she welcomed her first child, a daughter, in November (2018).
"It was definitely a special experience being pregnant when filming," she smiled. "It will always be a special movie because of that; she is in every scene."
The 42-year-old is now relishing her greatest role to date - as a mother to her little girl.
"Let's not f**k that one up!" she quipped.
Although Diane is learning everything about parenting for the first time, Norman is already an old hand at fatherhood - he shares 19-year-old son Mingus with his ex, supermodel Helena Christensen. The former couple dated from 1998 until 2003.