The trove of 33 letters, written from 1950 to 1964, to Reverend Joseph Leonard, who lived in Dublin, will be auctioned on June 10 at Sheppard's Irish Auction House in Durrow, Ireland.
Kennedy, who died in 1994, was a woman who kept her private life well hidden. She never granted an interview after she left the Washington, D.C.
In the letters, some written on her trademark blue stationery, Kennedy reveals her fears that John F. Kennedy would have extramarital affairs, as well as her deep despair after he was assassinated in 1963.
The auction house's Phillip Sheppard told the Irish Times the letters come from "private sources." They are, he says, "her autobiography for the years 1950-1964." And the estimates are as high as $1.65 million for the collection.
Kennedy, then 21, was introduced to Leonard, then 73, by one of her family members during a visit to Dublin in 1950. The two met only twice over the course of 14 years. (Leonard died in 1964.)
In 1952, she wrote to Leonard that she was "terribly much in love" with her then-fiancé, a New York stockbroker named John Husted. But he wasn't the only man she had feelings for. She also noted that she'd met JFK, then a 35-year-old congressman.
"I think I'm in love with [him]" she wrote "If he ever does ask me to marry him, it will be for rather practical reasons – because his career is this driving thing with him."
Family and InfidelityShe also wrote candidly of her concerns that JFK might stray. "He's like my father in a way," she wrote to the priest in the early days of her romance with the future U.S. president. "Loves the chase and is bored with the conquest and once married, needs proof he's still attractive, so flirts with other women and resents you. I see how that nearly killed Mummy."
Kennedy also confided how their relationship gave her "an amazing insight on politicians – they really are a breed apart." And she described him as being consumed by ambition "like Macbeth."
Still, she admitted, the power was intoxicating. "Maybe I'm just dazzled and picture myself in a glittering world of crowned heads and Men of Destiny," she wrote in 1953, "and not just a sad little housewife."
After his 1963 assassination, Kennedy wrote in her black-edged stationery, "I always would have rather lost my life than lost Jack," before adding: "I think God must have taken Jack to show the world how lost we would be without him, but that is a strange way of thinking to me."
It's an extraordinary discovery about the glamorous and guarded first lady. "We know next to nothing about Jackie's private thoughts about anything," Pamela Keogh, author of Jackie Style, tells PEOPLE. "The letters are fascinating because they show, clearly and honestly, she went into her marriage with wide open eyes and she knew exactly what was going on. Even from her early twenties. You also see what the private Jackie was like as a friend – how warm and open she could be with someone she trusted, and how thoughtful and vulnerable she was when she let her guard down."