Monika Fourie, 34, threw hot water from a cup and then a kettle onto Hannah Stokes in 2015, causing burns to her head, neck and chest.
Polish-born Fourie was convicted by a jury of causing grievous bodily harm with intent following a trial at Plymouth Crown Court in October.
Passing sentence, Judge Paul Darlow said: “It was a repeated assault on the same victim in two parts, quite separate.
“The throwing of boiling water from a cup you were holding, and then returning and picking up the kettle and throwing that over Hannah Stokes.”
The judge said he did not believe Fourie had carried out a premeditated attack on Miss Stokes.
“It involved the use of a weapon and involved, in my judgment, the intention of causing a more serious injury than was inflicted,” he said.
“The throwing of boiling water out of a cup was not a spontaneous act and it followed the boiling of a kettle and the absence of any intention to make a cup of tea.
“What followed added to excruciating pain she had already suffered. She was cornered and begging you to stop.
“You were pulling her hair up, trying to expose her face and you poured the kettle of boiling water over her.
“That’s what I mean when I say your intention was to cause more serious injuries than you did.”
Fourie broke down in tears when she was led away in handcuffs from the dock to begin her sentence.
Family and friends, including her mother and brother, who sat in the public, were also crying as she was led away.
The court heard Miss Stokes had begun an affair with Fourie’s now ex-husband, South African-born Royal Marine Wouter Fourie, in the summer of 2015.
Days before the attack, Mr. Fourie had confessed to the extra-marital relationship and the defendant had left their home in Plymouth to stay with a friend, but unexpectedly returned on the night of October 24.
Miss Stokes had been invited round by Mr. Fourie to watch South Africa play in the Rugby World Cup on television and was sitting on the sofa when the defendant returned.
Fourie said she was going to make a cup of tea but instead poured water from a cup over Miss Stokes.
She then returned to the kitchen, picked up the kettle and went back to the lounge, where Miss Stokes was screaming and grabbed her by the hair and tipped the remaining contents of the kettle over her.
Miss Stokes was treated for second degree partial thickness burns down to her navel and had surgery for a perforated eardrum.