Maybe Shen Biran still can't bring herself to think about it, but there's no way around it, this is exactly what he has to think about.
If he doesn't think about how Gu Qingyuan and Lu Xiao see it, then he simply can't go through with this.
Her relationship with Lu Xiao still explains everything: she can't really let go of him. If she could truly let all of this go, then perhaps the feelings they once had could also be given a clean, final explanation.
Yet what's happening now is no longer something they can clarify, and their relationship is only going to become even more complicated.
Even though Shen Biran no longer believes in Lu Xiao, from Lu Xiao's point of view, she also no longer believes in him that much.
She doesn't believe Lu Xiao will give up on Gu Xiaonuan, but what happened in the past makes Shen Biran feel very sorry for Lu Xiao, makes her feel that if she doesn't feel sorry for him, she'll be the one abandoning everything.
