New Clues Reveal How Marie-Antoinette's Secret Love Affair Was Doomed

What really ended the passionate—and illicit—love affair between Marie-Antoinette of France and the handsome Swedish aristocrat who may have fathered two of her children?

A new book, I Love You Madly—Marie-Antoinette and Count Fersen: The Secret Letters, offers clues to the split, which ended the nearly 20-year affair between the last French queen and diplomat Axel von Fersen.

Secret letters, many of which used code names and even invisible ink, capture the couple’s passionate exchanges. “I live and exist only to love you—adoring you is my only consolation,” von Fersen wrote in one of the letters, which have been compiled by historian and author Evelyn Farr. Marie-Antoinette, caught in a loveless marriage to King Louis XVI from age 14, wrote to him on January 4, 1792, “I love you madly and never, ever could I exist a moment without adoring you.”

But Farr detected that soon afterward there was a “change in tone” that offers a clue of an impending split. In her book, which is out next month in the U.S., she notes the fading intimacy between the two and she shows how “external factors”—such as the coming revolution in France—derailed the romance.

Fersen tried to rescue Marie-Antoinette from her opulent palace, Versailles, several times and take her to Belgium—but she refused to go and he took it badly, Farr establishes.

At some point in early 1792, they parted. “He went off and brooded and sulked and then tried to get back with her in April 1792,” Farr tells PEOPLE. “If the monarchy hadn’t fallen, I think they would have been all right.”

Farr believes the Queen must have told him that there was no way of keeping the relationship going. “I don’t think she realized how he was going to take it. He said, ‘I will write to you through the secretary.’ There is a complete change of tone. He had an attempt at reconciliation, but it was too late and they were at war and letters couldn’t get through. It tormented him for years,” says Farr.

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The author, who has charted their union via detective work using the letters—her book is the first time the code tables have been in print—adds, “I think their physical relationship was compromised simply because they were afraid of being caught by the [Marquis de] Lafayette. He was a secret revolutionary leader but was guarding the royal family. Everyone thought he was a constitutional monarchist but he was the enemy.

“He was desperate to force a divorce on Marie-Antoinette, lock her up in a convent and take away the children. That would have finished the royal family if you were a republican.”

Fersen subsequently fell for Eleanore Sullivan because of the “cooling” of his relationship with the Queen, Farr believes. “[Marie-Antoinette] kept putting him off. There is a letter he had written to his sister saying he had been miserable for four years, and I have dated that back to 1791,” she notes. “It was felt Sullivan was a very poor substitute, but it may have got back to Marie-Antoinette.”

Based on her research, Farr says she has compelling evidence that Marie-Antoinette’s son Louis Charles and daughter Princess Sophie—both of whom were thought to be the children of Louis XVI —were in fact fathered by Fersen.

Like their mother, who was executed at age 37 on October 16, 1793, after her trial at the hands of revolutionaries, both children met tragic ends: Louis died at age 10 after being imprisoned by the revolutionaries who overthrew the monarchy, and Princess Sophie died of illness just weeks before her first birthday.