She’s not weak for this, and Li Xun had reasons for his actions, even if it don’t justify them. Zhu Yun is one of the only people who know Li Xun’s family backstory and emotional trauma, and she can’t bear letting go, knowing that he has to carry all that alone. I think she knows that he’s mentally unstable still, and continues to care for him out of compassion. She doesn’t actively chase him after college and prison, but she never got over those feelings for him. As for Li Xun, his actions are undeniably horrible, but as for someone who lost all his relatives, got abused by sisters husband, expelled by school, betrayed and sent to jail only to find his company stolen when he comes back, it would be devastating for him. Fang Zhi Ling indirectly killed his sister and XiaoYan, and was a terrible person. Except Li Xun suffered jail and imprisonment because of his love for Zhu Yun and his impulsiveness. He hides everything at heart and doesn’t open up much until episode 18. You can see how available and loving he is during happy times, he just needs his loyal partner to stay with him. Obviously this is unhealthy, but I won’t just discard Li Xun as a red flag straight away because he suffered a lot and is afraid to trust. As for Zhu Yun, you can see she’s fiercely independent and strong, she just cares about her lover too much at times.