Shen Mochen knew Han Muxue liked him. It's often said that girls are very sensitive, but many boys are just as sensitive, especially someone like Shen Mochen who had a rich experience of being liked.
The reason they became what they were now was simply owing to the guilt they felt towards each other, especially Han Muxue.
So, all he needed to do was dissolve her guilt. Let her think he was a worthless jerk and that Xia Qingrou's decision to break up with him was the right one. This way, she wouldn't carry an emotional burden.
Shen Mochen deduced that if he did this, in just a few days, the relationship between Xia Qingrou and Han Muxue would be restored to what it was before. Moreover, his confession would likely boost Han Muxue's self-confidence about him as well.
However, the only downside to this method was that it would damage his own image and interests. It was a true case of self-sacrifice for the benefit of others... not something he would typically do.
