A woman drowned her daughter in a bath so the three-year-old's "spirit could be at peace", she told a court.
Claire Colebourn told police that moments before she killed Bethan in October 2017 the girl repeatedly touched her face and told her she loved her, Winchester Crown Court heard.
Mrs Colebourn told the court her daughter had been "suffering" after her parents broke up and Bethan would be "a lot safer in heaven".
She denies a charge of murder.
Mrs Colebourn told jurors her husband Michael - the chief executive of marine interiors firm Trimline - had moved out of the family's home in Fordingbridge, Hampshire, a month before.
Bethan was found dead by her grandmother at the house in Whitsbury Road on 19 October, the court heard previously.
The court heard Mrs Colebourn attempted to take her own life afterwards and was revived by paramedics.
She told the court her emotions were being "hammered" after her relationship broke down with her husband.
She said: "When... you see your beautiful little girl suffering because she feels for her mummy, then she's going to be a lot safer in heaven than she is anywhere near her father.
"Because her spirit can be at peace and Michael would not let us be at peace."
Breaking down in the witness box, Mrs Colebourn said her "head just went bam" on the night Bethan died.
The former sixth form college teacher was asked by prosecutor Kerry Maylin if she had intended to kill Bethan and then kill herself.
"Sadly yes," Mrs Colebourn replied.
"I was in a very emotional state. I thought that I would be with Bethan in heaven."
'Utter intensive fear'
She said she had believed her husband had been having an affair, had been financially controlling and had monitored her laptop and phone.
Ms Maylin previously said the fears over the affair were "unfounded".
After Mrs Colebourn left the stand, the defence said its case was closed.
Earlier the court heard the defendant only confessed to the killing in a third police interview, six months after Bethan's death.
Jurors were told Mrs Colebourn told detectives in that interview: "It was pure and utter intensive fear at the time. Fear of Michael's emotional abuse.
"My only aim was keeping Bethan safe...the only way out was for both of us to go away."
Mrs Colebourn woke Bethan in the early hours and ran a bath, the court heard.
"She knew I wasn't right because she kept coming up to me... put her hands on my cheeks and told me she loved me," the defendant told detectives in the police interview.
"Then I drowned my own daughter. I drowned my own daughter."
The trial continues.