Nesbitt was in New Zealand filming The Lord of the Rings prequel The Hobbit in June (12) when he heard his 79-year-old mum May had passed away, and he jetted back to Northern Ireland to be with his family.
The actor has now opened up about his grief, admitting he is upset his ill mother missed out on seeing "how much more content a person" he had become.
Speaking on Sky Arts' Living the Life TV show, he says, "There was a crossover when she knew something was wrong. What I hate most is the memory of her being frightened.
"Now the grieving process has begun and I think about her when she was well."
The interview is set to air in the U.K. later this year (12).