Although no explanation was offered as to why she harmed her pet, Glasgow Sheriff Court was told that she was an "attention-seeker".
On one occasion her employer gave Bretman an evening off, then correctly predicted that within a few hours the dog would suddenly become ill and be brought back to the surgery requiring emergency treatment.
In what is understood to be the first prosecution and conviction of its kind of an owner harming their dog in such a way, Sheriff Joan Kerr found Bretman, of Giffnock, Glasgow, guilty of injecting her own dog with insulin resulting in her requiring immediate treatment to "avoid coma or death".
The spaniel was re-homed after the allegations against Bretman came to light and the court heard she had since returned to good health.
Her former owner started working as a veterinary nurse in 2011 for Pet A&E, a clinic in the Kinning Park area of Glasgow which provides care for animals outside normal working hours.
Her former employer Lesley Herd grew suspicious after the dog needed emergency treatment on several occasions after collapsing, twitching and vomiting. Every time, tests showed a low glucose level.
Herd told the court: "The dog was fine between episodes so I really didn't know what was going on with the dog at all, we couldn't understand why she was having these episodes."
She added that initially Bretman did not want blood samples sent to a vet school for testing but eventually agreed to it. However, after volunteering to take the blood herself, it never arrived at the institution.
Herd added: "Because of the pattern of collapse and low blood glucose on each occasion, and the fact that the dog was normal between episodes, I was suspicious insulin had been administered to the dog."
The court heard that after Bretman was given an evening off on June 2013, the dog collapsed and they came in for treatment, as Herd predicted.
The veterinary nurse was later suspended and sacked from her job and the clinic contacted the Scottish SPCA over its concerns.
She was charged with causing her pet unnecessary suffering, including collapse, convulsion and seizures, by injecting it with insulin on June 23, 2013.
Bretman denied in court that she "took a dislike to the dog", and claimed Florence was her "companion". She will be sentenced next month.
The charge carries a maximum penalty of a £20,000 ($35,731) fine or a year in jail.