Killer Dad Chris Watts Is 'Tormented By His Past' in Prison, Says Source

One year ago today, Chris Watts did the unthinkable.

On August 13, 2018, the Colorado dad killed his 15-weeks-pregnant wife and their two young daughters. He first strangled wife Shanann, who was 15 weeks pregnant with the couple’s third child. Then, with her body in the back of his truck, he drove to a remote oil work site with his sleeping daughters — Bella, 4, and Celeste, 3 — in the backseat.

Before burying Shanann in a shallow grave, Watts said he smothered Celeste in the backseat, right next to Bella. After dumping Celeste’s body in an oil tank, he returned to smother Bella, who begged for her life. He then stashed her body in another oil tank.

A source familiar with Watts tells PEOPLE that he has finally taken responsibility for his actions. “He has nothing to do but think,” says the source. “He thinks about what he did every day. He is tormented by his past and the mistakes he made.”

At the time of the murders, Watts was having an affair with a coworker, Nichol Kessinger. Kessinger cooperated with police and said she thought Watts was separated while they were dating.

Watts didn’t immediately admit to his actions.

According to an arrest affidavit obtained by PEOPLE, Chris allegedly said that after he told Shanann he wanted to separate when she got back from a work trip, he saw her strangling their 3-year-old, Celeste, while their 4-year-old, Bella, lay “blue” and apparently lifeless on her bed.

In response, Chris allegedly claimed, he “went into a rage” and strangled Shanann.

But authorities didn’t buy his shocking explanation. As the case headed for trial, Watts, now 34, pleaded guilty to all three killings. He is currently serving five life sentences without the possibility of parole.

In a Feb. 18 interview from inside prison, Watts gave chilling details of the murders to authorities from the Colorado Bureau of Investigation.

He said his wife Shanann “may have been” praying as he strangled her to death inside their bedroom. He said he then drove his wife’s body to his job site, where he buried it in a shallow grave as his daughters waited in the car. Then he smothered both girls.

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During the confession, he said that whenever he closes his eyes, he hears his oldest daughter’s final words begging him not to kill her.

“Daddy, no!,” Bella Watts, 4, screamed as her father moved in to smother her with the same blanket he’d used to kill her sister Celeste, 3, moments before.

“I hear it every day, when Bella was talking to me,” Watts told police. “When she said, ‘Daddy, no!'”

The source tells PEOPLE that Watts has a long road ahead of him. “He’s left alone with his thoughts for 24 hours a day, 7 days a week. Every day. Birthdays. Holidays. Special occasions. He’s there in jail, in his cell, thinking about what happened. He’s in his own private hell.”