Susan Smith 'Cries For Her Children' Every Christmas Since Murdering Them, Says Source

Christmas has come and gone, and Susan Smith spent the day thinking about the two sons she killed in 1994, a prison source tells PEOPLE.

“The holidays fill her with guilt and regret,” the source, who has known Smith for several years, tells PEOPLE. “She cries for her children every Christmas. She knows she did the most horrible thing imaginable, and it tears her up every day. She’s a tortured soul.”

Smith, now 48, is in prison for the rest of her life at Leath Correctional Institution in Greenwood, S.C. She works as a wardkeeper assistant, and interacts sparingly with other guards and inmates.

Her only respite: a Bible that she keeps in her cell, along with a photo of the two sons who she killed in 1994.

Related Video: Mom of Slain Children in SC Murders Asks Mercy for Ex-Husband

Smith’s brushes with law enforcement began on October 25, 1994, when she told police that she had been carjacked by a black man who had taken off with her two young sons still in her car. For nine days, she made tearful pleas for their safe return.

But it was all a lie.

As her story began to unravel, Smith admitted that there was no carjacker, and that she had let her car roll into a lake with Michael, 3, and Alex, 14 months, still strapped in their car seats.

Her motive: She was secretly dating a man who didn’t want children. The story became international news, even landing on the cover of PEOPLE. Smith was convicted of two counts of murder and is serving a life sentence. She is not eligible for parole.

Now, 25 years later, Smith has spent more than half of her life in prison — and records obtained by PEOPLE show that her incarceration has been marred with disciplinary infractions.

In the past nine years, Smith has been disciplined at least five times for various infractions, including self-mutilation and the use and possession of narcotics or marijuana. Each infraction has resulted in the loss of privileges, including loss of visitation, canteen and telephone privileges.

Twice in 2010 and once in 2015, she was disciplined on drug charges, losing privileges for more than a year.

In 2000, Smith was disciplined for having sex four times with 50-year-old prison guard Houston Cagle. He pleaded guilty and spent three months in jail. The following year, a prison captain, Alfred Rowe, also pleaded guilty to having sex with Smith and was sentenced to five years probation.

“No one trusts her to be alone with a guard,” the prison source told PEOPLE in October. “She spends most of her time in her cell or working, but when she’s being transported, there is always a male and female guard with her.”

But Smith claims she has been misunderstood.

In 2015, Smith wrote a letter to The State. “I am not the monster society thinks I am,” she wrote. “I am far from it.”

“Something went very wrong that night. I was not myself,” she continued. “I was a good mother and I loved my boys. There was no motive as it was not even a planned event. I was not in my right mind.”

Smith’s ex-husband — and the father of the boys — told PEOPLE in 2010 that he has never fully recovered from the pain. “There’s always this nagging and gnawing heartache,” David Smith said, “It’s there every day, even if I’m not always conscious of it.”

The prison source tells PEOPLE that Smith’s horrific past is always on her mind. “She doesn’t talk about her boys much, but when she does, it’s always with a lot of sadness. She doesn’t expect anyone to forgive her, and she doesn’t look for pity. She’s still in mourning — and she’ll never stop mourning for the rest of her life.”

According to the jail, Smith is not permitted to give interviews. A call to her former attorney has not been returned.