Harry, 36, confirmed that he and his older brother, 38, were "on different paths" in 2019. Things allegedly escalated when Harry and his wife Meghan Markle announced in early 2020 that they were taking "a step back" as senior members of the British royal family.
The Duke and Duchess of Sussex, yearning to work independently, currently reside in Santa Barbara, California, with their son Archie, 1.
"It's certainly been a roller coaster for everyone involved, especially the brothers, who are vowing not to let things get this out of hand in the future and have learned a lot from this difficult experience," the source said.
The insider also insisted there has been "a lot of rejoicing" from the family, especially from their grandmother the Queen, as they work towards reconciliation.
"[William and Harry] are looking forward to spending time together in person the moment it's safe to travel," said the source, adding they will likely reunite in England.
Rumours of an alleged royal feud came to light in 2019, when British best-selling author Katie Nicholl claimed that since Markle, 39, and Harry announced their engagement in 2017, William grew increasingly worried for his brother and his whirlwind romance with the American actress.
"William was quite concerned that the relationship had moved so quickly," Nicholl claimed in the TLC documentary Kate V. Meghan: Princesses At War?
"And being close to Harry, you know, probably the only person close enough to say to Harry, 'This seems to be moving quickly. Are you sure?' And I think what was meant as well intended brotherly advice just riled Harry."
"Harry is hugely protective of Meghan," Nicholl continued.
"He saw that as criticism, he interpreted that as his brother not really being behind this marriage, this union. And I don't think things have been quite right ever since."
Harry hinted at an alleged rift with his older brother in ITV's 2019 documentary Harry & Meghan: An African Journey.
"Part of this role and part of this job, this family, being under the pressure that it's under, inevitably stuff happens," he said at the time.
"But look, we're brothers, we'll always be brothers. We're certainly on different paths at the moment but I'll always be there for him and I know he'll always be there for me."
In November 2020, royal historian and biographer Robert Lacey told Fox News that William had reason to be concerned about his sibling's romance with the former Suits actress.
"Let's make clear that William's reservation wasn't whether Meghan was right for Harry, but whether she was right for the royal family and royal life," Lacey explained at the time.
"When it came to Kate [Middleton], William was slow and cautious. He followed duty. Harry immediately fell in love and discovered a new destiny for himself, a new meaning in his life. William saw Meghan as a self-made woman who created her own celebrity and wealth in the process. The royal family, on the other hand, is in the business of representing values.
"Royals are expected to know their place," Lacey continued.
"And Harry, of course, was expected to marry a nice girl named Henrietta or Gabriella who lived in the provinces of Britain in the countryside and settle down. But Meghan and Harry both made it clear they wanted to do so much more than that. And let's just say there are things that you can and cannot do when it comes to running the royal family and what it stands for."
Lacey said that while William and Harry "speak on the phone from time to time", it will take a while for their relationship to heal.
"I think it would always have happened that Meghan and Harry would live on the other side of the world," he said. "I think Meghan and Harry will remain based in North America. Unless the brothers can find a way to reconcile, they will have to go their separate ways and keep mutual respect for each other.
"We do know that in July 2021, both brothers have committed to being in Kensington Gardens for the unveiling of the statue dedicated to their late mother, Princess Diana.
"The world will judge then. But there is a lot of work to be done."