Singer Billy Joel flew into a jealous rage after a former lover spurned him to marry another man — then drowned his sorrows in a sea of booze.

That’s the untold story behind the “Piano Man’s” shocking entry into a Connecticut rehab facility for a 10-day stay.

Joel “went off the deep end” after up-and-coming TV news star Trish Bergin got engaged, reveals a source close to him.

The 53-year-old songwriter telephoned Trish incessantly, jammed her voice mail with pathetic messages and even demanded that her fiance “step aside” for him!

“Billy just couldn’t stand having Trish say ‘no’ to him,” the source told The ENQUIRER.

“But it’s his own fault. Billy could have had Trish — if only he hadn’t constantly fooled around with other women.”

Trish, now 31, met Joel four years ago when she was a newscaster at a local Long Island, N.Y., station.

“Trish was living with another man then and she later married him,” the source says.

“But Billy didn’t care. He started trying to win her over any way he could.

“When her marriage turned rocky, Billy really turned on the charm. After she got divorced, Trish finally agreed to a date.

“Suddenly, Billy was flying her to London on the Concorde to have lunch with Elton John. The next six months were a whirlwind of attention.

“Trish loved it — and Billy.”

But in fall 2000, Trish discovered Joel had been carrying on behind her back with a singer in Arizona, the source said.

Then she walked in on him in bed with another woman — and learned about even more women, including a backup singer in Billy’s band.

Desperate to keep her, Billy proposed with a $40,000 3-carat antique diamond ring.

“But Trish said ‘no.’

She knew Billy was incorrigible about women,” said the source.

This past spring Trish announced her engagement to lawyer Randy Weichbrodt. About a week later she went to Billy’s house to return his ring.

“Billy was a wreck. He’d been holed up, drinking nonstop,” said the source.

“For the next few weeks, he called Trish countless times a day on her cell phone and at her office.

“She changed her cell phone number three times to avoid him and his pitiful messages. She finally had to shut off her voice mail at work.

“He even left a message for Randy, saying: ‘Step aside. I’m determined to marry Trish. And when I want something, I get it!’ “

Meanwhile, Joel unraveled in public.

In March, he postponed his tour with Elton John. In early June he pulled out of the Songwriters Hall of Fame dinner after he wrecked his Mercedes.

Two days later, a pregnant Trish — who’ll coanchor TV’s “Inside Edition” in August — got married.

That same day, Joel checked himself into Silver Hill Hospital in New Canaan.

“I hope Billy did it to get the help he desperately needs,” the source told The ENQUIRER.