I kinda understand Sage's hangup. She is in her last year. She has had what was for her a steady relationship for at least 2 years (as far as we know). She is probably thinking about life after college (marriage, children). And now everything is turned upside down with Chad cheating. I can understand how she would still hang on to that vision she had in her head. It would be wiser to just let it go, but it's hard to stay rational in such a situation. We have the luxury of 3rd eye vision and a long time to think. She only has had a week since seeing the proof of the cheating.
Sage needs time to process and come to grips on the situation. If she simply lets go, it would feel to her like the last 2 years were a waste.
Yeah, as hard as it might be for some people to wrap their heads around why Sage is so hung up on Chad, we don't know the full story. Not just about their relationship, but about Sage too. She may have issues related to her past that make it difficult for her to give up on Chad.
Perhaps she has a fear of abandonment, or an insecurity about not being loved, and so she clings to Chad because with him, she doesn't feel so alone and/or if he had been affectionate towards her in the early days then this feeling of being loved is what she holds on to and why she puts up with him. But now, the MC is starting to show her affection that may be greater than Chad, and he's often there for her when she needs him, which is why her feelings may be starting to change.
I'm sure we'll find out (hopefully soon) what the deal is exactly and, knowing DPC, it will probably be something that will fuck with the emotions.