The high-profile horsebreeder wrote a letter to her friends and family in an attempt to reassure them that they could not have done anything more.
Up to 1,000 mourners from the world of racing and beyond attended the private funeral service at St Ultan's, down a boreen in Lucy's picturesque native village of Rathcore, Enfield, Co Meath.
Ms Stack and her husband, trainer Fozzy Stack, had been "racing's golden couple".
But mourners were told that Lucy had "suffered silently from a young age".
The emotional service was planned by Lucy right down to the hymns and readings – she said she wanted it to be a "thanksgiving" for her life.
Chief mourners were Mr Stack, Lucy's mother Denise Foster and her siblings Jessie and Nick, along with her parents-in-law, trainer Tommy Stack and his wife Liz.
Among those present were Tipperary TD Michael Lowry, trainers Ted Walsh, Tom Taaffe and Dermot Weld, and jockey Charlie Swan.
Chief celebrant Rev Janice Aiton told mourners that Lucy had been described by her mother as being "beautiful inside and out", while Lucy had called her mum "the strongest, most caring mother ever".
Bravely, Fozzy Stack read aloud his wife's letter, which had been sent to friends. It was dated February 1, the day before her body was found at their home in Golden, Co Tipperary.
"To my great friends, thank you for the fun times, the laughing, dancing and giggles. I've been so blessed with the friends I've had," it said. "You're a very special bunch and I need you to know that what has happened is no reflection of how you were as my friend.
"No call could have changed this. No chats over wine.
"Sadly, this is just my fate. I know it is selfish and cowardly but for me it is my only option. I can't face this world any longer. I'm just not strong enough. Too much has happened."
The letter, signed "Friends forever, Lucy", caused many mourners to break down in tears.
Rev Aiton told mourners that Lucy had loved nothing more than to be outdoors with her beloved dog, Lola.
She spoke of Lucy's "energy and warmth of character", her enterprising spirit that had taken her off to Australia to work in the bloodstock industry there and of her love of people, which led her to work in public relations.
After the service Lucy's coffin was taken for burial in the adjoining graveyard.