Julia Knight, 56, had denied trying to kill Irene Robson, 81, but admitted administering the insulin at Horton Hospital in Banbury in October 2014.
During a trial at Oxford Crown Court, Knight said she did it so her mother would not be discharged from hospital.
Prosecutors said it was a "premeditated, cold-blooded attack".
The trial heard Knight forged a prescription for insulin, which she collected at a Sainsbury's store in Banbury.
Knight told jurors she had wanted to make her mother a "little bit poorly" and described it as "a stupid, stupid, thing to do".
Updates on this story and more from Oxfordshire
She also told the court she had attempted suicide in 2006 by injecting herself with insulin.
Det Insp Craig Kirby of Thames Valley Police said: "This is an incredibly sad case.
"It was only following a large scale police investigation, when the weight of evidence became overwhelming, that Julia Knight finally admitted injecting her mother with insulin, something she had repeatedly denied during a number of police interviews.
Adrian Foster, chief crown prosecutor for Thames and Chiltern Crown Prosecution Service said: "It is impossible to imagine a more vulnerable victim.
"Knight embarked on a premeditated, cold-blooded attack on her mother by illicitly injecting her with a potentially lethal dose of insulin while she was an inpatient on a ward after suffering a fall at home.
"The insulin, obtained by Knight via a forged prescription, caused Irene's blood sugar concentration to drop to a lethal level and she fell into unconsciousness, but hospital staff managed to save her life.
"There is no doubt that as a qualified practising nurse and with her medical knowledge of the effects of unprescribed insulin that Knight intended to kill her mother."
Ms Robson died in September but the trial heard her death was not relevant to the case.
Knight is due to be sentenced on Friday.
A nurse who injected her mother with insulin as she lay in hospital after a fall has been found guilty of attempted murder.