EXCLUSIVE: Inside Prince William's Nightmarish Introverted Behavior — And the Huge Change That Sparked 'Disturbing' Change in Royal's Conduct

But insiders say the separation – which lasted only a few months – deeply affected William, revealing a rare glimpse of his private struggles.

A former royal staffer said: "That time really weighed on William. He wasn't himself – much quieter, more withdrawn. You could tell something wasn't right. He remained courteous and composed, but there was a heaviness about him that everyone noticed."

The source added: "He never spoke about it directly, but it was clear to all of us it had to do with Kate. The atmosphere around him completely changed."

And former butler Grant Harrold, who served the royal family for more than a decade and was working at Highgrove House during William and Middleton's split, recalled the same transformation.

"He went very quiet. He became a little bit withdrawn. I didn't see a huge amount of him," Harrold says in his memoir The Royal Butler: My Remarkable Life in Royal Service. "I just knew he wasn't himself 100 percent. I knew he wasn't himself."

At the time, William faced intense speculation about his relationship and growing pressure from the media to propose. Friends say the separation was as much about protecting Middleton as it was about buying the couple time away from the spotlight.

"The media pressure on Kate was overwhelming," said a royal aide. "They were still very young, and the level of scrutiny was intense. Stepping away for a while was the right move – it gave them both room to breathe."

Harrold added he always believed the pair would reunite.

"Kate was always the one (for William)," he said. "Even when they were apart, you could tell she was never far from his thoughts."

He added staff at the time quietly hoped for a reconciliation, describing Middleton as "lovely, very bubbly and very talkative."

Those close to her say Middleton, now 43, handled the period with her usual grace despite the public intrusion.

"People think she's shy, but she's not," Harrold said.

"She was warm and approachable – very down-to-earth. If you offered her a drink, she'd already be helping herself. She never acted like she was above anyone."

Insiders suggest the brief split with William helped strengthen Middleton's bond with the future king in the long run.

"It really opened their eyes," said a longtime royal observer.

"William came to see just how much he relied on Kate – not only as his partner but as the person who keeps him grounded. She gave him a sense of calm and stability he didn't have without her."