"(Her death) was never real," Liam said in a poignant interview with CBS' Anderson Cooper for 60 Minutes. "It still kind of isn't.
"There's periods now in our New York residence when I hear the door opening, especially the first couple of years… anytime I hear that door opening, I still think I'm going to hear her."
The 61-year-old actor told Anderson how he arrived at a Canadian hospital to be told by doctors that his wife was brain dead following her accident.
"She was on life support… I went in to her and I told her I loved her, said, 'Sweetie, you're not coming back from this, you've banged your head'," he said.
"She and I had made a pact — if any of us got into a vegetative state that we'd pull the plug. That was my immediate thought, 'OK, these tubes have to go. She's gone,'"
Liam and Natasha were married in 1994 and have two sons together. He said he kept her life support on for a short time so that her friends and family could say goodbye.
The Irish star went on to say that while his grief is now less immediate, it's very much a part of his life. "It hits you. It's like a wave," he said. "You get this profound feeling of instability… the Earth isn't stable anymore and then it passes, and it becomes more infrequent. But I still get it sometimes."
Liam takes comfort in that fact that Natasha lives on in others. "Donated three of her organs, so she's keeping three people alive at the moment. Her heart, her kidneys and her liver," he said. "It's terrific and I think she would be very thrilled and pleased by that."