Butting in... :) I think you're confusing Justin having to be in a different physical location for his career with them breaking up or with Justin "leaving" Brian in the relationship sense of the word. But they're not the same thing. Being physically apart for periods of time isn't easy, but it doesn't equate to breaking up or to Justin leaving Brian in any way other than being bodily in a different city.
Putting Brian aside for a minute, Justin has enough faith in their love and in his commitment to the relationship that he won't perceive it as being anything more than two busy professionals who lead demanding lives, sometimes in different cities, but not in a way that results in Brian being gutted every time Justin gets on a plane.