Laura Newlands, 15, from Rhyl, Denbighshire, died at home on 8 August 2011.
Her father, Andrew, told an inquest in Ruthin on Monday Laura "was getting picked on" at Rhyl High School and teachers were not stopping it.
Laura told social workers in early 2011 she was having "suicidal thoughts".
The coroner asked Mr Newlands if he felt he could have done more to hide his medication, given the fact his daughter had reported having suicidal thoughts.
He replied: "I should have done more. I feel part of the blame is mine for not listening to her - trying to do things different to what I had done.
"I have had a lot of time to think about it. I have blamed myself on numerous occasions. It was my fault."
Earlier in his evidence, he talked about problems Laura was having at school.
"She were getting picked on in school. Girls in the same class as her would starting picking on her in front of the teacher. The teacher would do nothing about it."
He said he and his wife had been to the school several times and it was agreed his daughter could leave five minutes early at the end of the day to avoid other pupils, but it made very little difference.
The inquest continues.
A teenage girl who took a fatal overdose of her father's medication was being picked on at school, an inquest has heard.