Hunky Brooks Forester is nursing a secret heartbreak – and that’s the real reason he had second thoughts about “The Bachelorette” Desiree Hartsock!

A source tells The ENQUIRER that Forester was left devastated when the love of his life cheated on him, and he grew terrified that Desiree would do the same thing.

Viewers were shocked by his dra­matic decision to walk out before the show’s finale.

“I broke Brooks’ heart and he’s never forgiven me,” his former sweet­heart, 28-year-old model Erin Reese, told The ENQUIRER. “I dated him for nearly two years, and about 18 months ago we called it quits.”

But an insider close to Forester claims there’s more to the story.

“Brooks loved Erin very much,” said a family insider. “They were talk­ing seriously about getting married, but then he found out that Erin was cheating on him. Brooks broke up with her and never looked back.”

And when Forester, a 28-year-old sales and marketing rep, saw Desiree canoodling with his rivals on “The Bachelorette,” he couldn’t take it, said the source.

“When Brooks watched all the other guys falling over Desiree, and saw her kissing two or three of them a day, it tore open the wound in his heart from Erin,” said the insider.

“The memories of Erin cheating came flooding back, and he couldn’t handle any more betrayal.”

Looking back, Erin says losing Forester broke her heart too.

I loved Brooks, and even though, I tried to make up with him, he refused to take me back,” she told The ENQUIRER. “When I realized that he would never forgive me, I moved out of the state to get over him.”

Forester had been tabbed as “the one” for Desiree, 27, for several weeks.

But a source says he flipped out after seeing her interact with Chris Siegfried, a 27-year-old Oregon mortgage broker (the ultimate winner), and Drew Kenney, 27, a digital marketing analyst from Arizona, while “The Bachelorette” filmed in Antigua.

“From his hotel balcony, Brooks could see Desiree in a hot tub, kiss­ing a guy and drinking champagne,” said the family insider. “Then the next morning, she was rolling around on the beach in a tiny bikini, and ended up sitting on top of another guy!

“Brooks knew what he was signing up for with ‘The Bachelorette.’ But at the same time, when he saw Desiree kissing other guys week after week, it was disgusting to him.

“Even if he gets sucked back into being with Desiree, I don’t see their relationship going the distance.”

Forester’s former flame agrees.

“Brooks is very serious about his rela­tionships,” Erin told The ENQUIRER. “I was surprised to hear he was go­ing on ‘The Bachelorette,’ because a person can’t fall in love in a matter of weeks, especially not Brooks.

“I’m sure Desiree is a nice girl, but knowing Brooks, I’m not surprised he walked away.”