The wedding you both planned
He thought that as long as he pretended not to know everything will be fine but tonight when he thought about Yu Dong might lose Xiao Hua and becoming an echo of his past self then he could no longer stop himself from letting her go after Xiao Hua. When Mu Yuxi had left him for Chu Cheng, he had felt like he was half dead, it was only Yu Dong's arrival that saved him from the misery that he suffered from, only when she married him and gave him home was he able to feel what it meant to be at peace.
But when he saw Yu Dong with the same desperation and pain that he had in his eyes when he was left by Mu Yuxi, he realised that he was too engrossed in himself that he completely forgot Yu Dong. She has stood up when she needed to take responsibility for him and saved him from a shameful death but he has subconsciously ignored everything that she wanted and wished to have.