Maryanne Jones said she was devastated by a phone call from police after the remains were found on Tuesday in Glenrothes.
Her son Kenneth, then aged 17, has not been seen since November 1998 after he walked out of the family home without any possessions.
She said: “It has been really difficult. Everything that has happening now brings it all back.
“The phone call from the police was devastating. You don’t know if the remains are him or not and it brings it all back.
“If it’s not him it just leads you back to square one and I need to know. I need closure.
“It’s been 17 years and you feel people have forgotten about him because it’s been so long.”
Kenneth’s disappearance on the morning of 3 November 1998 remains a mystery.
Searches provided no evidence of his whereabouts, while images of him appeared on milk cartons in an bid to trigger help from the public.
A computer-generated image was also released in recent years, showing an impression of how he might look today.
The last time his mother saw him was the night before he left as he went upstairs to bed, having just taken the bin out at the request of his father.
An only child, Mrs Jones said her son was a happy boy who stayed out of trouble. Before his disappearance there was nothing to suggest he was suffering from depression or may wish to leave home.
Mrs Jones dismissed subsequent rumours about him joining groups of travelling people, which emerged in the years following his disappearance.
She said: “Kenneth wasn’t street wise, he didn’t go about in gangs. He was quite settled and would normally go to his uncle’s to play the computer.
“He didn’t go to pubs drinking or out to the dancing. Nothing came of the rumours of him joining up with Travellers. There was no evidence to support that, just a lot of rumours.
“We’ve never had any sign or clue as to what happened.”
Following her son’s disappearance Mrs Jones divorced Kenneth’s father and moved house, but still lives close to the former family home in the town’s Ancrum Court.
She added: “He didn’t take any clothes or money or anything. It’s terrible. You just live in a trance.”
Police have confirmed that a group of teenagers discovered human remains in woodland in Riverside Park, Glenrothes, on Tuesday afternoon.
Chief Inspector Stevie Hamilton said: “A number of teenagers discovered what they believed to be human remains. They immediately came to Glenrothes police station to report the find.
“It’s a difficult area to work in and we’ve put a cordon on to protect the scene.”
He added: “The scene is particularly difficult. It’s densely overgrown and as you would expect we have to approach it very carefully to preserve and not destroy any evidence if we have a crime scene.”
With a number of local families with missing relatives alerted, Mr Hamilton also said that keeping them informed was of paramount importance.
However, he said that he would not rule out the possibility that it may be a person unknown to police.