Mother guilty of murdering daughter Bethan, three, by drowning her in bath
Claire Colebourn, 36, did not react as the jury foreman read out the unanimous verdict at Winchester Crown Court and there was silence in the public gallery.
The five women and six men deliberated for around two-and-a-half hours on Friday after an eight-day trial.
She will be sentenced on Monday morning.
The court heard Colebourn hit "rock bottom" after her high-flying husband Michael ended their 16-year-relationship.
She woke the couple's daughter Bethan at their home in Fordingbridge, Hampshire, in the early hours of October 19 2017, led her to the bath and drowned her by holding her body down under the water, before making multiple suicide attempts.
Colebourn was discovered by her mother about 14 hours later, in a diabetic coma.
The ex-sixth form teacher initially denied all memory of the incident, but later recalled how Bethan agreed to have a bath in the middle of the night at her mother's instruction, telling police: "Sadly, my little girl trusted me completely."
Colebourn, who met Michael, now the chief executive of luxury marine interior company Trimline, at university, admitted she killed Bethan but said she only wanted to "save" her from the little girl's father.
But jurors rejected her account, delivered during a fraught and emotional day's evidence from the witness box on Wednesday.
Kerry Maylin, prosecuting, said: "Bethan was three-years old, she was a much loved daughter of Claire and Michael Colebourn, but she was found dead at her home.
"Claire Colebourn, when Bethan was found, was also in the house and she was at the time suffering a diabetic episode.
"Bethan died because she had been put in the bath at home and held under the water, the act was completed by her mother."
The prosecutor said that just over a month before Bethan's death, Colebourn and her husband separated and Mr Colebourn had moved out.
Miss Maylin said their relationship had been difficult and the defendant went on to make "unfounded accusations" on Facebook and in an email sent to his work, that her husband was having an affair with the company's finance director.
In the email to Trimline, she wrote: "He has been having an affair with his finance director at work, everything has been pre-planned, of course the finances.
"They are aiming to take over the business and set up a new life together."
Miss Maylin said Colebourn also told friends she had visited Ms Futcher's home looking for her husband's car.
The lawyer said Colebourn also became worried, without foundation, that Mr Colebourn was monitoring her computer and arranged for an IT firm to supply her with a new laptop and wifi at her home.
She said that within minutes of receiving the new computer in October, she began searching for suicide-related websites.
Miss Maylin said that following her arrest, she told medical staff she had tried to kill herself the previous day.
The prosecutor said a doctor described how she was "only emotional when discussing the fact Michael Colebourn had left her and her daughter and her father had done the same to her mother".
She later wrote in a letter intercepted at a hospital she was staying in: "In my eyes I saved her, everything over those days is a blur."
She later told police: "I am responsible for Bethan's death because she drowned and I am responsible for it. Bethan drowned because I was there, I held her under the water."