Frances Shand Kydd's revelation was made during a special memorial mass for her daughter at St Columba's Cathedral, Oban, the Scottish Roman Catholic cathedral where she worships.
In a powerful tribute, Mrs Shand Kydd also talked of the many private moments she secretly shared with Diana in places all over the world.
She wavered only once during her tribute. She said: "I know that grief has no agenda and no timetable. I also know that the ache will never go. But there will be gentler, kinder days."
Mrs Shand Kydd, 61, said she had found "indescribable comfort" from being with the thousands of people mourning Diana.
She continued: "Much has also been written about Diana's relationship with me, supposedly authoritative articles, based on no possible knowledge or basis of facts.
"Such pieces made us both wince, but also gave us joy that we often met in many different places without anyone ever knowing."
She told, too, of the 15,000 messages of condolence and regret she has received.Reuse content