Ex-soldier Christopher James Finn, 35, also had to join a gang in a bid to stay safe at a jail where accessing mental health support is difficult, Brisbane Supreme Court was told.
Justice Peter Applegarth said it would be a "disgrace" if Finn's situation did not change, ordering Queensland Corrective Services provide a pre-sentence report.
They must explain how they will manage Finn's mental health treatment and his anticipated transition to another prison once he is sentenced.
Justice Applegarth described Finn's situation at Woodford Correctional Centre as "deeply unsettling".
"Any citizen who read what this man went through wouldn't want him sleeping on the floor with his head next to a toilet in a maximum security prison where he has to choose between gangs for his protection," he said.
"If he spends the next 12 months in the prison that he is in, in the situation that he is in, without the help that he needs, it would be a disgrace."
Finn served in the army for 10 years and was diagnosed with PTSD after witnessing "significant traumatic events" during four tours.
Finn's life began "spiralling downhill" in August 2020 when he turned to drugs as his mental health deteriorated, defence barrister Alastair McDougall said.
Finn pleaded guilty to a total of 38 mostly drug-related charges in Supreme Court following a 14-month crime spree that ended when he was found in possession of a loaded revolver in public in August 2022.
"It's a chicken and egg scenario in terms of his mental health decomposition either leading to drug use or drug use contributing to his mental health decomposition," Mr McDougall said.
Finn's mental health was at risk of going backwards at Woodford prison, he said.
A report from Finn's psychologist said she had experienced difficulties with Corrective Services when trying to organise video conferences.
Justice Applegarth described the report as a "grave cause for concern", saying there were also problems being addressed by the ongoing Royal Commission into Defence and Veteran Suicide.
"I can't solve the problems of veterans' mental health today - that's why we have a royal commission," he said.
Justice Applegarth said prison mental health services have an enormous burden to treat inmates.
However, he said a person who had served his country with distinction was now relying upon mental health services in custody "which notoriously are inadequate to treat the prison population that have collectively massive mental health problems".
Justice Applegarth on Thursday adjourned sentencing to a later date, ordering Corrective Services provide a report on how they would handle Finn's rehabilitation, safety and security.
"It doesn't mean he gets some special break because he's a veteran," he said.
"Unless he gets the attention that (psychologist's) report suggests he desperately needs in custody right now his mental health situation is likely to become worse not better and that can't serve the long term goal of community protection."
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