Giving evidence against Ralph Clarke the woman, now in her 50s, said she did not want "to go to my grave knowing that I let him get away with it".
The former lorry driver is alleged to have committed 31 offences against three children, including one as young as seven, between 1974 and 1983.
Mr Clarke, of Holly Lane, Erdington, Birmingham, denies the charges.
The alleged victim told jurors she went to police because she was unable to cope with the "guilt" of not making a complaint.
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The witness told the court she had decided to complain to police in August last year after suffering nightmares and flashbacks.
She told Birmingham Crown Court she had hidden abuse she suffered as a young child from school friends.
Responding to questions from Mr Clarke's barrister Darron Whitehead, the alleged victim said: "I hid it from everybody because you feel dirty and ashamed - I felt as if I had done something wrong."
When Mr Whitehead said Mr Clarke denied abusing her, the woman added: "I am not going to my grave knowing that I let him get away with it; I am not taking that guilt to my grave.
"He's a monster. I couldn't cope with it any more."
Mr Clarke denies 17 charges of indecent assault, 12 offences of indecency with a child and two attempted serious sexual offences.
The trial continues.
A 101-year-old man accused of child sex offences was described as a "monster" by an alleged victim.