A woman accused of attempting to murder her husband's lover told jurors he was "dishonest and disloyal" and had become obsessed with a woman he met at work.
Alethea Foster said her husband John was denigrating and secretive
Alethea Foster, 61, told Cambridge Crown Court that in her misery she felt unloved, unwanted and "raw inside".
The prosecution claims Mrs Foster tried to kill Julie Simpson, 45, after finding emails written by her husband.
Mrs Foster told the court her husband of 35 years had lied to her and was "denigrating, secretive and boring".
Miss Simpson, a mature student from Beckenham, south London, was stabbed 17 times in the hall of her Cambridge college in October 2005.
Mrs Foster, a retired foot specialist, told the court on Friday she and her husband had not had sex for 15 years - he had told her that he was impotent and there was no treatment.
Mrs Foster stabbed Julie Simpson 17 times
"I found it a hard thing to have a sexless marriage and I was so sorry for him," she told the court.
She said she could not believe her husband, a former political journalist with the BBC, would have sex with another woman after telling her he was impotent.
When she voiced her suspicions that he was having an affair with Miss Simpson - a family friend - she said he had always fobbed her off and gave the impression she was being unreasonable.
Mrs Foster, who also denies causing grievous bodily harm with intent, said she had not intended to attack Miss Simpson - also a former BBC journalist - but meant to kill herself in the student's room.
The prosecution said Mrs Foster took a kitchen knife from her home which she used to attack Miss Simpson.
The court has heard Miss Simpson was blinded in one eye in the attack.
The trial continues.