The tragic story you’ll never forget — behind  BRITNEY’s sudden and bizarre career moves.

Quit "The X-Factor" before getting fired? Check. Bust up with “boyfriend” agent co-conservator? Check. Plot a Vegas-style extravaganza? Check.

How now, bayou? What gives?  

Here’s WHAT: Britney is in mourning – deep, deep mourning for her beloved gran’ pappy.

Brit’s Christmas holiday was marked by heartbreak – her beloved grandfather June Austin Spears died on Dec. 16 at the age of 82 at his Louisiana home.

“Britney called him ‘PaPa June’ and she was very close to him,” an insider said. “Her grandpa tried to keep her centered, even at the height of her mental problems.

“He wanted her to get away from Hollywood and the fast lane. He knew it was no good for her even though she had tremen­dous success. He urged her to move back to (her hometown) Kentwood, settle down and get married again, and have more babies.

“Britney said, ‘I always told PaPa June I would do that one day, and now he’s not going to be around to see me keep that promise.’”

Britney, 31, was devastated when her mother, Lynne, called to break the news of PaPa June’s passing.

“Britney was inconsolable,” revealed another source. “She knew that he wasn’t getting around as much, but she had no idea that he was near death.”

June, an Air Force Sergeant in the Korean War, is the father of Britney’s dad, Jamie.

The Spears patriarch was a police officer for the Baton Rouge (La.) City Police during his working years. According to sources, June loved to take Britney fishing and squirrel-hunting when she was young.

“Britney started a scrapbook of all her child­hood photos and the letters that PaPa June sent over the years,” the source said. “She’s even written a song about him.

“Britney’s been in tears ever since he died – and now she’s even talking about moving back home to fulfill PaPa June’s dying wish.”