They were planning for their wedding; he had told her he would commit (they broke up once before I think during Bang Bang and they got back together) and even announced it during an interview in 2015 but changed his mind again. That’s the reason why she stayed for so long, he kept on lying to her and in the end getting cold feet and wasting her time before dating her “friend”. He also did nothing to stop his parents from treating her like a pest and humiliating her publicly. And let’s not forget the vacation photos he leaked. She said in an IV with GQ that she wasn’t as close to his family as she’d like and wanted to hang out more with them (but they clearly didn’t). She also pretty much explicitly admitted that he was cheating on her.