The prosecutor said that just over a month before Bethan's death, Colebourn and her husband separated and Mr Colebourn had moved out.
Colebourn was concerned that her husband - now the chief executive of luxury marine interior company Trimline - wanted custody of their daughter, despite him never asking for it.
In a police interview read out in court, she said: "If he had just given me a little bit of money to buy a little house for me and Bethan, he could have seen her whenever he wanted to.
"I was at rock bottom, I guess. What did I do? Protect Bethan. He made me feel useless. Her life without me would have been horrific.
"Bethan drowned and I am responsible for her death. She drowned and I am responsible, fully responsible. I was there, I held her under the water.
"Why did I do that? Because I didn't want her to go anywhere near her father. Whether that was right or wrong, that is desperate times for you.
"I can't tell you my feelings at the time because I can't remember them. I now realise it was pure and utter fear. Fear and anxiety.
"Fear of Michael. I was fearful of abuse, emotional abuse. I thought if he could do that to me, I didn't want him to do it to Bethan.”
A biology teacher drowned her three-year-old daughter to “protect” her from her father after the breakdown of their marriage.
Claire Colebourn, 36, hit "rock bottom" after her high-flying husband Michael, who she met at university, ended their 16-year-relationship, a court heard.
She woke their daughter Bethan at their home in Fordingbridge, Hampshire, in the early hours of October 19 last year, led her to the bath and drowned her by holding her body down under the water, before making multiple suicide attempts.
She was found guilty of murder by a unanimous verdict at Winchester Crown Court yesterday and will be sentenced on Monday morning.
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."