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.

The 23-year-old, Ulster's Player of the Year, made his international debut for Ireland in the Six Nations game against England at Twickenham in February.
McCloskey still had a year remaining on his previous contract but had no hesitation in signing a long-term deal.
"I grew up in Ulster and it was always a dream to wear the jersey," he said.
"Despite some strong interest from other clubs, it was a fairly straightforward decision since all of my family and friends are here.
"The supporters, the facilities we have and the quality of the coaching were all factors in my decision.
"We had a disappointing finish to the season but I believe things are promising for the future and I'm excited to be part of that."
McCloskey has accumulated 42 appearances, scoring five tries, since making his debut for the province in February 2014 as a replacement in the Pro12 fixture against the Dragons.
Ulster Director of Rugby Les Kiss described McCloskey's expressed delight at McCloskey's decision to extend his stay.
"Stuart has been one of our standout performers all season. A major strength of his is his gain-line busting runs, which we used to great effect this season.
"In addition to his powerful carrying ability, his deft handling and decoy lines have also caused havoc for defences."

Dead Men Risen, written by Toby Harnden and published by Quercus, took the prize at a ceremony in Westminster and was the judges' unanimous choice.
It focuses on the death of Lt Colonel Rupert Thorneloe in 2009.
It was published in amended form after the first print run was destroyed by the MoD.
The judges said the book "takes us into the hearts and minds of the Welsh Guards in a way that is both interesting and visceral".
"It challenges every citizen of this country to examine exactly what we're asking soldiers to do in Afghanistan," the panel continued.
"Rather than offering easy answers it lets the soldiers speak for themselves."
Other awards presented included a posthumous honour for Christopher Hitchens, whose final book, Arguably, was included on the long list for the top prize.
Hitchens' widow Carol Blue accepted the award on behalf of the Vanity Fair writer.
The journalism prize was awarded to Amelia Gentleman for her work in The Guardian, while the blog award went to Rangers Tax Case - an online commentary on the ongoing financial problems at the historic Scottish football club.
The writers of the blog said they aimed to "provide the details of what Rangers FC have done, why it was illegal and what the implications are for one of the largest football clubs in Britain".
The winners were chosen from shortlists of six books, six journalists and seven bloggers, whittled down from longlists of 17 books, 12 journalists and 18 bloggers.
Each of the winners received a Â£3,000 prize.
A 101-year-old man accused of child sex offences was described as a "monster" by an alleged victim.
[NEXT_CONCEPT]
Ulster centre Stuart McCloskey has signed a contract extension which will see him remain at Kingspan Stadium until the summer of 2019.
[NEXT_CONCEPT]
A book about the death of a British officer in Afghanistan, once pulped by the Ministry of Defence, has won the Orwell Prize for political writing.