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Leeza Tracey Ormsby, 37, was arrested in a police sting in February but it took months for them to charge her with possessing a half-smoked marijuana joint she admits having in her handbag.
She is not contesting the charges that carry up to 12 years in jail and will ask the court to treat her as a drug user deserving a lighter sentence.
The Sydney resident apologised to the judges at her trial on Thursday.
"I knew the law but I broke it," the former Paddington shop assistant said.
"From this incident, I got a valuable lesson.
"I will learn about myself and about the world."
Ormsby told the court she only stumbled on the drug bust on February 12 when she went to collect her swimsuit and laptop from the villa where friends were staying.
"When I arrived, there were two men and when I tried to open the door of the villa, I couldn't," she told the court.
"After I managed to get it opened, I was pushed inside then came a lot of policemen. They showed me police identity."
Asked by a judge what she thought then, Ormsby said: "Oh God, this is trouble".
Psychiatrist Ni Nyoman Hanati told the court Ormsby was a person "who had never felt happy".
Ormsby was one of 13 children and given up for adoption at age six to a family that frequently left her alone, she said.
She had been admitted to a clinic for depression, and had a heroin overdose six years ago.
Hashish and MDMA were seized from the villa where Ormsby was arrested but no one has ever been charged in connection to those drugs.
The court will hear sentencing submissions next week. Ormsby remains in custody in Kerobokan jail.