Too late, again.
Harker stood, frozen in a moment of time, as Mina fell; slowly and elegantly. Were it not for the trail of crimson blood following her, Harker would say Mina was the most beautiful thing in Creation.
But at that moment, she was lifeless and would never share her laughter with him again. The bullet made sure of it.
So when he returned to that moment in time, it was only natural for him to try and save her from her fate. A lifetime later, he was, somehow, at that moment, anew.
But he was too late.