Former science teacher Claire Colebourn was handed a life sentence at Winchester Crown Court with a minimum term of 18 years after being convicted of murdering her daughter Bethan.
The court heard Colebourn hit "rock bottom" after her high-flying husband Michael ended their 16-year relationship.
She woke their daughter at their home in Fordingbridge, Hampshire, in the early hours of October 19 2017, led her to the bath and held her body under the water before making multiple suicide attempts.
The ex-sixth form biology teacher was discovered by her mother about 14 hours later in a diabetic coma.
After the sentencing, Mr Colebourn said his "beautiful daughter" had been taken from him in a "cold and callous manner at the very hands of the one other person that should have protected her and kept her safe".
In a statement, he said: "The one thing in my life that gave me purpose has gone.
"I desperately miss being a daddy - we would have such great times together; Bethan's laugh was infectious and her energy was endless.
"There is not a second in the day that goes by that I am not thinking about her.
"Bethan was my world and being her daddy made me so proud. I miss her so much."
Colebourn, dressed in a black jacket and wearing glasses, rubbed her temples with her hands, shook her head and looked at the floor as details of her crimes were given to the court.
The 36-year-old did not react when handed the sentence.
Mrs Justice Johannah Cutts said: "She (Bethan) was clearly a beautiful little girl who was full of life.
"She had everything to live for.
"You were her mother, you were responsible for her care and her well-being."
Colebourn planned to end Bethan's life, set an alarm to "carry out the act" and altered her will the next day.
During the eight-day trial, the jury of five women and six men were told she 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 admitted killing Bethan but said she only wanted to "save" and "protect" her from her father - who Colebourn had met at university and is now the chief executive of luxury marine interior company Trimline.
The pair separated just over a month before Bethan's death and Mr Colebourn had moved out.
Kerry Maylin, prosecuting, told the court their relationship had been difficult and the defendant had made "unfounded accusations" her husband was having an affair with his company's finance director, claiming the pair were planning to take over the business and start a new life together.
Colebourn went looking for her husband's car and became convinced he was monitoring her computer so bought a new one and changed her internet connection.
Within minutes of receiving the new computer in October, she started searching for suicide-related websites, the court heard.
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".
In a letter intercepted at hospital, she said: "In my eyes I saved her, everything over those days is a blur."
Colebourn 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."
Ms Maylin said Bethan was particularly vulnerable given her age and had trusted her mother.
Mrs Justice Cutts said Mr Colebourn was "in no way responsible" for Bethan's death.
Karim Khalil QC, defending, said Colebourn appeared to have a personality disorder but this was disputed among experts.
He told the court she "sustained and maintained" her account and never sought to "deflect attention" over what happened to her daughter.
The NSPCC said it was a "desperately sad case" and Colebourn "failed her daughter in the worst possible way".
She has already spent nearly a year in custody and this will be deducted from the minimum term she must serve before facing the parole board who will determine whether she is ever released.