I think that Bryon's change towards Cathy reflects the challenges of growing up in That Was Then, It Was Now.
Let's start off by tracing what might be Bryon's feelings towards Cathy. We know that he does have strong feelings for Cathy. These feelings cause him to become different than Mark. One way he is different than Mark is because he develops empathy. An example of this would be how he feels bad for M&M, something Mark does not feel. The feelings that Bryon feels for Cathy play a large part in his separation from Mark.
When Bryon has to call the police on Mark, it represents one of the most difficult things he has to do. He is filled with guilt over what he has done. As a result, he is not entirely certain that he has done the right thing. When he testifies against Mark, it cuts the bond between them and Mark will never be able to forgive Bryon.
Bryon's feelings towards Cathy have helped him develop an emotional identity that he did not have before. However, as a result of these feelings, he has broken off from the life he once knew. In many ways, Bryon breaks up with Cathy "to get even for Mark." Perhaps, we can make the argument that he breaks up with Cathy as a way to show some loyalty to Mark, while he struggles with the fact that he committed the greatest act of disloyalty by turning him in.
As you can see, little is clear here. Growing up is shown in a very complex manner. I would say that Bryon still has feelings for Cathy, but he cannot bring himself to loving her because of the pain he harbors for what he has done to his brother. In this case, hurt is stronger than love. The emotions he felt for Cathy played a major role in why he did what he did to Mark, so continuing to feel love for her is like a further act of betrayal against Mark. Feelings like happiness and joy are in the past for Bryon, representative of the "that was then, this is now" idea in his life.