Not every "barely 18" relationship is healthy, and a storyline can explore toxicity without endorsing it. However, lazy writing often falls into these traps:

When writing these romantic storylines, embrace:

This trope is a staple, but at 18, it matures. It stops being about the thrill of rebellion and starts being about the "savior" complex. The storyline usually involves one partner trying to "fix" the other before they destroy their future.