Alyssa Bustamante, 18, had pleaded guilty to second-degree murder and armed criminal action in the October 2009 slaying of Elizabeth Olten in their small Missouri town. In her diary, she described the experience as “pretty enjoyable.’’

Bustamante had been charged with first-degree murder. By pleading guilty to the lesser charges, she avoided a trial and the possibility of spending her life in an adult prison with no chance of release.

Bustamante was 15 when she confessed to strangling Elizabeth, repeatedly stabbing her in the chest and slicing the girl’s throat. She led police to the shallow grave where she had concealed Elizabeth’s body under a blanket of leaves in the woods.

With her hands shackled to her waist and her feet shackled together, Bustamante rose and faced Elizabeth’s mother before she was sentenced. She struggled to compose herself. “I know words can never be enough and they can never adequately describe how horribly I feel for all of this,’’ she said. “If I could give my life to get her back. I would. I’m sorry.’’

The teenager’s defence attorneys had argued that Bustamante’s use of the antidepressant Prozac had made her more prone to violence. They said she had suffered from depression for years and once attempted suicide by overdosing on painkillers.

But prosecutors noted Bustamante had dug two graves several days in advance. Missouri State Highway Patrol Sgt. David Rice testified the teen told him “she wanted to know what it felt like’’ to kill someone.

Prosecutors also cited journal entries in which Bustamante described the exhilaration of killing Elizabeth.