Theron, 39, spoke candidly about her relationship with Penn, 54, to Elle UK.
"Putting aside that he's my partner, he's the love of my life; for the first time, I felt that my work was really bleeding into my life and that made it hard," she reveals in the magazine's latest issue. "There were moments where I was incredibly unfair to him. And moments where I felt like… he was incredibly unfair to me."
But it's those "unfair" times that have made her "realize that no matter how complicated it gets, the priority is the relationship."