I didn't see this in the cinema and I just watched it on DVD. I do not understand your praise of this film. Mostly, the plot is predictable. In good romances, this is not a problem because the characters are likeable. Here it is the opposite. Nadine is annoying. Erwin is boring. When the plot is not predictable it is unbelievable. Nadine says she can't drive. Suddenly, she steals her mother's car and is Baby Driver. The end is underwhelming: Darian hugs Nadine and changes her personality.
You seem surprised by the poor box office for this film. For better or worse (mostly worse), mainstream Hollywood movies are made for teenage boys. Try selling this: "The main character is a whiny prick tease".
Is there a director's cut where Nadine jumps off an overpass? If you want a good teen romance, watch Sixteen Candles.