Claire Colebourn, 36, showed no emotion as jurors returned their verdict at Winchester Crown Court.
The court had heard she hit “rock bottom” after her high-flying husband Michael ended their 16-year-relationship.
She woke the couple’s little girl, Bethan, at their £400,000 home in Fordingbridge, Hampshire, in the early hours of October 19, 2017.
She then led the youngster to the bath and drowned her by holding her body down under the water, jurors were told.
Colebourn, who used to teach Sixth Form will be sentenced on Monday morning.