In her first network TV interview since the actor's death in 2008, Williams tells Nightline's Cynthia McFadden that she's "found meanings around the circumstance, but the actual event...I still can't find it."
Williams and Ledger split in September 2007 after three years together. Ledger was found dead in his apartment four months later from what was later ruled an accidental overdose from prescription medication. The couple's daughter, Matilda, is 5.
Williams, 30, says she's found comfort in the book The Year of Magical Thinking by Joan Didion.
"It didn't seem unlikely to me that he could walk through a door or could appear from behind a bush. It was a year of very magical thinking, and in some ways I'm sad to be moving further and further away from it," she says.
"In a strange way, I miss that year," the Blue Valentine star adds, "because all those possibilities that existed then are gone."
Watch the full interview: