lostalgia
noun
Otherwise pleasant memories tinged with regret that something important has been lost forever.
Cleaning out his closet, Jon found an old box that had never been unpacked through several years' worth of moves. Opening it up, he found the Star Wars novels he’d obsessively collected and read as a teenager. For a moment he was excited, and then he felt a strong sense of lostalgia as he recalled that this particular volume had been a gift from his mother who had died two years ago.