I have a vague recollection of watching a drama where the following happens- I'm assuming it's a cdrama because I know I've seen way more low-ish budget chinese youth dramas and I believe it falls into that category.

I remember that the FL lived with her dad, and her mom came back to visit here and there. She was happily convinced her mother was away for work and would one day move home. And then it is revealed that her parents are actually divorced, and hadn't told her. I think her mom actually has a whole new family but I'm not 100% sure on that part (I think a new husband, potentially even a new daughter too?)

Other than that specific aspect of the storyline I don't remember a lot, I think it's probably a youth romance cdrama based in school / early university and that there is a ML.  The drama was probably released 2017-2022, I don't tend to watch older dramas. 

Can anyone help? Have I hallucinated this whole thing? 

No, you didn't hallucinate, I remember watching something like that, but I am not recollect he name of the drama.