"It's amicable," one source says of the breakup, which was first reported by In Touch. "It was just a normal breakup. It just ran its course."
When exactly things fell apart is unclear because…
Cornelsen was with Knight just two weeks ago at the opening of Parade, a stage musical starring the former Grey's Anatomy star. It's Knight's first project since leaving the ABC prime-time medical drama.
We spotted Cornelsen, who is 16 years younger than the 36-year-old Knight, attending the show with another Grey's alum, Melissa George.
Knight's romantic future isn't the only thing now up in the air for the actor. He was scheduled to star on Broadway next year in a revival of Lend Me a Tenor, but a source says those plans could be put on hold because Parade may be moving to Broadway.
Either way, it looks like Mr. Knight will be temporarily relocating from his L.A. home to NYC. And I'm told Cornelsen never intended to join him for the move.
Find out what we think of T.R. Knight's performance in Parade here.