Say Colter does the right thing and stops seeing Christina -- he still has to deal with the man's other friends and family and with his job as a teacher. Colter is a soldier, so unless he's also been getting a degree in education on the side, we're pretty sure that he's about to engage in some Kindergarten Cop-style hijinks. He's going to have to fake his way through basically every interaction he has with everyone, like the guy from Memento, but with fewer tattoos.
Our finest team of mathematicians were paid to whip an intern while he calculated the film's precise genetics.
Add this to the fact that, per the movie's ending, the Source Code (the machine that allows Colter to quantum-leap into Sean's body) was actually creating alternate timelines with every attempt to save the train. This means that, in theory, an infinite number of Jake Gyllenhaals will be creeping out an infinite number of Michelle Monaghans in an infinite number of universes, forever.