Virginia Maria Beerens, 59, pleaded guilty to one charge of attempted murder in the Christchurch High Court.
Beerens, a dental assistant, stabbed Benjamin Han in the chest with a flick knife.
Crown prosecutor Chris Lange said Beerens expressed her frustration to police that she had failed in her attempt to kill Han.
Beerens and Han were in a relationship from 2015 to the middle of 2017, Lange told the court.
The couple rekindled their relationship in January this year, however, Beerens became jealous of Han seeing another woman.
On the morning of January 22, Beerens was drunk and decided to kill her partner.
She travelled to Han's work where she saw his car, then drove to his flat in Riccarton.
A flatmate, who recognised Beerens, let her inside.
She entered Han's bedroom and burned notes and gifts from the other woman on the bed.
After watching the fire ignite, Beerens took a black flick knife "which she knew her partner regularly sharpened", said Lange.
Beerens returned to Han's work to check whether flatmates had alerted him to the fire. Beerens saw his car was gone and returned to his flat.
When Beerens arrived at the flat, firefighters had arrived and were extinguishing the blaze.
Beerens approached Han at his front door, with the flick blade in hand.
After talking to Han, Beereens stabbed him "driving the knife into his chest as far as the blade would allow", said Lange.
Han fell to the ground bleeding and Beerens threw the knife into the garden.
Fire service staff restrained Beerens until police arrived.
Han was admitted to Christchurch Hospital in critical condition and underwent surgery for his injuries.
Beerens will be in police custody until her sentencing date on October 24. She faces up to 14 years imprisonment for the attempted murder of Han.