Susan Smith cries for her children every Christmas since she murdered them, source says

Dave Martin/AP

Christmas is over and Susan Smith has spent the whole 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. “Every Christmas she cries for her children. She knows she did the worst thing imaginable and it tears her apart every day. She is a tortured soul.”

Smith, now 48, will spend the rest of her life in prison at the Leath Correctional Institution in Greenwood, South Carolina. She works as a prison aide and has little contact with other guards and inmates.

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

Related Video: Mother of children killed in South Carolina murders pleads for mercy for her ex-husband

Smith's run-ins with police began on October 25, 1994, when she told police that her car had been stolen by a black man who drove off with her two young sons in the car. For nine days, she tearfully pleaded for her safe return.

But it was all a lie.

When her story came to light, Smith admitted that there was no car thief and that she had driven her car into a lake with three-year-old Michael and 14-month-old Alex still strapped into their car seats.

Her motive: She was secretly dating a man who didn't want children. The story made international headlines and even landed 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.

Today, 25 years later, Smith has spent more than half her life in prison – and records obtained by PEOPLE show that her incarceration was marked by disciplinary violations.

Over the past nine years, Smith has been convicted at least five times for various offenses, including self-mutilation and the use and possession of narcotics or marijuana. Each offense resulted in the loss of privileges, including visitation, cafeteria and telephone rights.

She was disciplined twice for drug offenses in 2010 and once in 2015 and lost her privileges for more than a year.

Susan Smith in August 2017 | South Carolina Department of CorrectionsSusan Smith in August 2017 | South Carolina Department of Corrections

Susan Smith in August 2017 | South Carolina Department of Corrections

In 2000, Smith was convicted of having sex with 50-year-old prison guard Houston Cagle on four separate occasions. He pleaded guilty and spent three months in prison. The following year, prison warden Alfred Rowe also pleaded guilty to having sex with Smith and was sentenced to five years probation.

“Nobody 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 at work, but when she is transported, there is always a male and a female guard with her.”

Michael (left) and Alex Smith | Dave Martin/APMichael (left) and Alex Smith | Dave Martin/AP

Michael (left) and Alex Smith | Dave Martin/AP

But Smith claims she was 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 father of the boys – told PEOPLE in 2010 that he never fully recovered from the pain. “There's always this nagging, gnawing grief,” David Smith said. “It's there every day, even if I'm not always aware of it.”

The prison source tells PEOPLE that Smith's horrific past is constantly on her mind. “She doesn't talk about her boys much, but when she does, it's always very sad. She doesn't expect anyone to forgive her, and she's not looking for pity. She's still grieving – and she's not going to stop grieving for the rest of her life.”

According to the prison, Smith is not allowed to give interviews. A call to her former attorney was not returned.