The father of Claudia Lawrence, missing, said that it was increasingly difficult to believe that the head of the university was still alive after almost 10 years without her.
Peter Lawrence said that the decade since his daughter's death from home in York had been "horrific".
Miss Lawrence was last seen on March 18, 2009 and the police think she was murdered, although no body was found.
Mr. Lawrence said in the past that he thought his daughter, who was 35 years old when she disappeared, is still alive.
But speaking to the Press Association from his home in York, he said he was now less certain.
"It is becoming increasingly difficult to believe that time is pbading," he said. "It's very difficult now."
Using one word to describe the last 10 years, Mr. Lawrence said, "Horrible".
And he said he did not know what happened to his daughter was the hardest part.
He said, "It just goes on, of course, because I do not know what happened, and it's not knowing who's always been the worst.
"I've always said that and, until we know what happened, it will continue, unfortunately."
Mr. Lawrence reported the disappearance of his "shy" daughter after her friend Suzy Cooper told her that she could not contact Miss Lawrence, a "pbadionate about texting".
Recalling how he felt when he realized something had happened to him, he said, "It was obviously a shock, it's more than worrying."
Mr. Lawrence went to his daughter's home in Heworth Road, York, to see if he could find her.
"I think I expected to find her lying on the floor in the house, but there was absolutely no sign.
"The house was like she was out working," he said.
"And nothing indicated that something unusual was happening in this house."
But as the hours turned into days, it became clear to him that something unfortunate had happened.
"Over the days, it became clear that this was not just the case. She would be back in a few days, "said the 72-year-old.
Peter Lawrence has been appointed OBE for his work with relatives of missing persons (Victoria Jones / PA)
Mr. Lawrence said he was surprised at the initial reaction of the police when he reported the disappearance of his daughter.
"In less than five minutes, two policemen were at home and a few hours later, in the afternoon, there were hundreds of police officers," he said.
Nine people were arrested or questioned on bail because of Miss Lawrence's disappearance, and the police submitted files to the Crown Prosecution Service regarding a number of individuals, but there was insufficient evidence to prosecute individuals. arrested.
Mr. Lawrence admits to being frustrated by the lack of progress in the case – particularly by the fact that the CPS has stated that he could not sue any of those arrested.
But he added that the police keep in touch when she receives new information.
"The problem is that they just need useful information," he said.
Lawrence said he has overcome the last 10 years "with the support of close friends and the community".
He used his experience to campaign tirelessly for the introduction of what is now known as the Claudia Act, which will allow families of missing persons for more than 90 days to take care of their business.
The law received royal badent in April 2017 and is expected to come into effect in July of this year.
He said: "We are almost here after so long. Claudia's law is to help many families – about 2,500 of them are waiting for her. "
He also found solace in the song with the Missing Persons Choir, which participated in the final of the British Got Talent in 2017.
He said, "We meet just to be together and sing new songs. Many people in the street say, "Oh, we know how you feel," but of course they do not know it. But the other members of the choir do it.
And close friends will be there to support him on March 18, which will mark a decade since the death of his daughter, and he said he would spend "silently".
With tears in his eyes as he pondered over the past 10 years, he said, "Sometimes it seems like an eternity, and sometimes it seems like it was not that far.
"I can not understand this at all, but that's how feelings disappear."