They just weren't in synch.

In a statement to last Thursday, reps for Justin Timberlake and Jessica Biel said the pair "mutually have decided to part ways." The harsh truth: Timberlake, 30, "had been aggressively trying to end things" with the actress, 29, since the Golden Globes Jan. 16, one source close to the singer tells the new Us Weekly, out Wednesday.

What's more, the pal adds, "he hasn't been happy with Jessica for close to two years." (The couple got together in January 2007, shortly after Timberlake's split from Cameron Diaz.)

"You'd be surprised at how soon the love was lost on this one," the friend continues. "Justin said he was miserable."

Timberlake finally fessed up to his feelings about the relationship shortly after his Jan. 31 birthday. "He just said he didn't want to marry her and it didn't feel right," another source says. Biel, the source says, was blindsided. "It just stung her in the heart."

But even in that final "talk," Timberlake didn't come clean about his rampant affairs with other women (including actress Olivia Munn, among others.) "He didn't admit the cheating, even though we all knew it," the insider says.

For much more on the big-time breakup -- why Biel never "believed" stories of his cheating, how Timberlake partied the day of the split announcement and more -- pick up the new Us Weekly, on stands now.