From left: Catherine, Duchess of Cambridge, Prince William and Princess Diana
REX USA (2)
Despite the overabundance of dignitaries in attendance on his 2011 wedding day
to Kate Middleton
, there was one person missing, though very much on Prince William
's mind: his mother, Princess Diana
"Very difficult" is how the prince recalls her absence
that day, as he reveals during an ABC interview
airing Tuesday night, as part of The Jubilee Queen with Katie Couric
"I sort of prepared myself beforehand so that I was sort of mentally prepared ... I didn't want any wobbly lips or anything going on," says William, 29. "It's the one time since she's died, where I've ... thought to myself it would be fantastic if she was here, and just how sad really for her, more than anything, not being able to see it."
Diana was killed in a Paris car crash in August 1997, when William was 15.
"I think she ... she would have loved the day and I think, hopefully, she'd be very proud," he tells Couric. "I'm just very sad that she's never going to get a chance to meet Kate."
, 27, also gets a chance to remark on his brother's wedding day and what it would have meant to their mother.
"I think she had the best seat in the house, probably," says Harry. "But yeah, she would have loved to have been there."