A wife referred to Prince Charles's affair with Camilla before stabbing her husband's former lover, a court heard.
Jurors heard Miss Simpson had been left blind in one eye
Alethea Foster, 61, quoted Princess Diana's "there are three of us in this marriage" before stabbing Julie Simpson, 44, jurors were told.
Mrs Foster, of Bromley, south east London, denies attempted murder and causing grievous bodily harm in Cambridge in October 2005.
Her trial at Cambridge Crown Court is expected to last until next week.
The court heard that Miss Simpson and John Foster, a retired BBC political journalist, had been involved in a relationship for more than 15 years.
Miss Simpson, of Beckenham, south London, said the affair had been over for two years when she was attacked at Lucy Cavendish College, Cambridge.
The undergraduate student lost the sight in one eye and suffered a punctured lung after being stabbed 17 times.
The attack happened after Mrs Foster, who had been married for 35 years, arranged a meeting with Miss Simpson to discuss the affair and had arrived with a kitchen knife hidden in her handbag.
She told the jury her role in Mr Foster's life had been minimal.
"I didn't feel like a third of a marriage. I thought that was a gross over-statement, I hardly saw the man."
She went on: "She asked me if I loved him and I said 'Yes, but it doesn't matter, I can walk away'. I was struck with the absurdity of discussing a man with whom neither of us was having sex. I was embarrassed.
Mrs Foster denies attempted murder
"When she was sticking the knife in me she was gleeful. When I was holding on to her arm I knew she was waiting for me to die. She was content and satisfied."
Miss Simpson, who had been a friend of the family, said she had urged Mr Foster to lie about their affair.
She said: "I didn't think that his wife should know that he loved another woman. I never thought that. I thought it would be devastating to her."