My wild theory: the twist is that Sungwoo was the real culprit behind her parent's "accident" and also the one to fake Taeyang's "affair," and he's a regressor too. But now, Taeyang is trying to protect Eunha from him bc he found out the truth in their last life and is also now a regressor (and possibly knows Sungwoo is as well or Sungwoo is and suspects Taeyang is too). And Sungwoo basically turned yandere after losing out in the last life.
Just saying bc both guys are so sus, and the author does a great job at using narrator bias to tell the story (reminds me of how the author of Cheese in the Trap uses the same to set up the thriller atmosphere in the story).