The 36-year-old actress recalled in an interview with the Wall Street Journal her heartbreak at selling the townhouse she and Ledger lived in as a family with daughter Matilda.
"At that time, I was inconsolable, because I felt, 'How will he be able to find us? This is where we lived, and he won't know where we are,'" Williams shared.
"And now I can't believe I thought that. Maybe what's making me cry is I feel sad for the person who thought he won't be able to locate [us]," she said. "The past -- you might be done with it, but it's not done with you."
Williams and Ledger started dating in 2004 after meeting on the set of Brokeback Mountain, and welcomed Matilda the next year. The pair split in 2007, just months prior to Ledger's death from an accidental overdose at age 28 in January 2008.
"There are loose ends," the actress said of life as a single parent.
"I think about work and how to do both all the time," she admitted. "I worry about the next job and when it's coming and will I be able to get it, but when you're looking at something, there's also the criteria of timing, the school calendar, the location, the duration and just where we're at as a family."
"How much does this work for me as a person, and how much does this work for my family? Sometimes they balance up perfectly, and sometimes they lean in one direction," the star confessed.
Williams had said in the April 2015 of Elle U.K. that she feels like she's "so good at raising a kid when [she's] not working." She appeared in the films Certain Women and Manchester by the Sea in 2016, and will star as Janis Joplin in a new biopic.