Guilt-stricken still struggles to forgive himself for pulling the plug on his wife, , in 2009 after the 45-year-old actress suffered a fatal injury while skiing — even though the Irish screen star once promised he’d never leave her languishing on life support, sources say.

According to a source, Liam, now 73, rushed to Natasha’s side at the New York City hospital where his wife of 15 years was flown after falling and hitting her head at a resort in Canada, and experiencing an excruciating headache.

“When he arrived, he found her on life support,” the insider says. “And when doctors told him there was no hope she would ever recover, he was faced with the most heart-wrenching decision of his life: Let her live dependent on machines, or keep the promise he made to her years ago — to disconnect her from the machines that were keeping her alive.”

The source states that Liam and Natasha — parents of two sons — had “the talk” years earlier, agreeing that if either were left incapacitated and relying on life support, the other would discontinue medical interventions.

The insider says that after doctors explained that Natasha had a hematoma, resulting in a traumatic brain injury, Liam felt he had no choice — even though it was the hardest thing he has ever done.

“Since that time, he’s tried dating other women,” the source shares. “But he keeps comparing them to his beloved Natasha. In his eyes, no one can compare with her — so he remains alone.”