Its definitely done on purpose. Hints are thrown all over that she knows more about Cecil's situations that she's letting on so her attitude makes you question the why of her stubbornness when she very obviously cherishes Cecil so much (she dresses in his colors unprompted, she writes letters to him when they are apart and takes proactive action in any issue she thinks will benefit Cecil regardless of her own character arc, like when she tried to turn her father into a criminal or when she made sure to get the other couples together so Cecil would have no competition). If you are frustrated with the story then that's the way it should be, because you are mirroring Cecil's own frustration too, and he is our narrator, so... You pretty much are seeing what he sees.
Yeah.. It's like: "what should I do to make you trust me? That everything will be fine"
I can't help but find Bertia annoying sometimes. When she forces him to act some way, it seems as if she doesn't see Cecil as the current Cecil. But like in this chapter when she acts considerate towards him it seems she does notice his changes. It's contradictory but I guess it's natural because she actually wants to be with the Cecil that loves her but doesn't believe that she should be with him.