An "urgent review" has been carried out by the Independent Inquiry into Child Sex Abuse into its service in Wales following a complaint by a victim.
The victim, who wanted to remain anonymous, has criticised the "poor level of support" he received from the truth project - the arm of the inquiry for victims to share their experiences.
He said he experienced delays and that his support worker broke down in tears.
An inquiry spokesman apologised to the victim.
The IICSA has since put measures in place to prevent it happening again, the spokesman added.
The victim spoke to BBC Wales less than two months since IICSA opened its Wales office in Cardiff, on 26 October.
He should have been contacted two weeks before he was due to attend the office to tell them about the abuse he suffered as a child.
Instead, the victim said he was contacted 48 hours before his appointment and the support worker who phoned him broke down in tears and told him of her own experience of abuse.
"Crying, she kept saying 'I've let you down, I've let the whole truth project down' and the conversation continued and she got quite hysterical," the victim said.
"She said 'I know how you feel - I've experienced trauma in my life'.
"I ended up counselling her."
He also said that he had concerns about other people who had come forward.
"This affects people in different ways. I feel quite strong. You don't know who is out there, people coming forward - they could be suicidal by the outcome and there's no help for them," he said.
Prof Alexis Jay, the fourth chairwoman of the inquiry, told BBC Wales in October that the inquiry was not in crisis.
However, solicitor Charles Derham, head of the abuse team at Verisona Law, disagreed.
"I think it's doomed. I think it's gone beyond the reach of any successful or positive outcome and I think that's very sad because it's potentially yet again another negative outcome from the result of their disclosures," he said.
The inquiry has been beset by controversy - including the three previous chairwomen resigning.
Prime Minister Theresa May set it up in 2014, when she was home secretary, to investigate allegations made against local authorities, religious organisations, the armed forces and public and private institutions in England and Wales, as well as people in the public eye.
The victim who has spoken to BBC Wales said he was abused by a youth worker in the late 1970s while attending a summer camp in Porthcawl.
BBC Wales has since learned that the alleged abuser worked as a teacher, employed by Cardiff council, and understands that a fourth person has come forward with more allegations about him.
South Wales Police has investigated but did not pursue the case because of the poor health of the accused, who is now in his 80s.
Three of the four men are pursuing a civil claim for damages against Cardiff council.
In response, Cardiff council said: "The local authority takes all allegations of historical abuse extremely seriously and they are thoroughly investigated.
"The council is unable to comment on specific cases whilst matters are ongoing."
A spokesman for the inquiry said: "We were very sad to hear about [the victim's] experience with us and we offer an unreserved apology.
"We thoroughly investigated his case and we have put measures in place to prevent this happening again.
"We carried out a clinical review across the inquiry, which highlighted exactly what went wrong in his case and we have offered to meet with him to explain what steps we have taken."