Dr Lore Hartzenberg gave the testimony ahead of the runner's sentencing for culpable homicide, and it was almost immediately characterized by the chief prosecutor as unbalanced.
Hartzenberg said the double-amputee runner had sometimes cried, retched, perspired and paced up and down during meetings in which she tried to assist him.
The testimony was part of an effort by the runner's legal team to persuade Judge Thokozile Masipa that Pistorius has suffered emotionally and materially for what he said was an accident and that he is remorseful.
Pistorius, once a celebrated athlete who ran in the 2012 Olympics, was charged with premeditated murder but Masipa instead found him guilty last month of the lesser charge of culpable homicide. Sentences for that conviction can range from a suspended sentence and a fine to as many as 15 years in prison.
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"We are left with a broken man who has lost everything," Hartzenberg said during her testimony.
Prosecutor Gerrie Nel criticized her findings, saying Pistorius would likely have the chance to rebuild his life.
Masipa drew criticism from some South Africans who thought Pistorius could at least have been convicted of a lesser murder charge on the grounds that he knew a person could die when he fired four bullets through a toilet door in his home early on Valentine's Day last year.
Steenkamp, a 29-year-old model, died in the hail of bullets, and prosecutors said Pistorius had opened fire in anger after the couple argued. The runner testified that he mistook Steenkamp for an intruder who was about to come out of the toilet and attack him.
"Some of the sessions were just him weeping and crying and me holding him," Hartzenberg said, describing Pistorius as overcome with grief and guilt that he had caused Steenkamp's death.
She said the shooting and Pistorius' lengthy and high-profile murder trial meant the athlete had also suffered severe loss. He had lost Steenkamp, his "moral and professional reputation," many of his friends, his career and his financial independence, she said.