Carol King-Eckersley's son was one of the 270 people who died in the explosion aboard Pan Am flight 103.
However, it was only this year that she found out that he was dead, a documentary team has uncovered.
King-Eckersley, 65, had given baby Kenneth Bissett up for adoption after becoming pregnant at 19.
Her father was a school headmaster and, she says, it would have damaged his reputation if it was known he had a pregnant, unmarried daughter.
After giving Kenneth up for adoption the teenager vowed not to look for him - but decided to seek him out earlier this year following the death of her husband.
An internet search showed his name on a remembrance page.
She said: "A total of 270 people died in that tragedy and one of those happened to be the only child I ever had.
"And I didn't even know it until last April.
"So it became a kind of double tragedy. I found him and I lost him on the same day."
Bissett had been a Cornell University junior studying with Syracuse University's overseas programme in London.
He had been due to fly home to New York a few days earlier but stayed in London for a 21st birthday party arranged by friends.
"I'm still in the semi-numb part after you lose a loved one," said King-Eckersley as the twenty-fifth anniversary of the bombing approaches. "Even though I didn't have him with me physically, he was always in my heart. I thought of him pretty much every day."
King-Eckersley told her story as part of a BBC documentary, Living With Lockerbie, which will be broadcast on tomorrow at 10.35pm on BBC Scotland and on Saturday, December 21 at 9.10am on BBC World News.