I think Yang Yang is telling himself to not say things that will push LiHuan away/hurt him. He is aware that what he is saying is hurtful but is unable to stop himself due to the since of betrayal.
wrong chapter sorry!!! In chapter 53 I think he is telling himself not to open up to Li Huan.
Talked about that in my discussion thread too ;D -> http://www.mangago.me/home/mangatopic/3496411/ I think it's YuYang telling himself not to open up to LiHuan because he doesn't believe people would understand him and he thinks no one would like him back </3

I really don't get the "Don't say it" parts in Ch 53. Anyone a clue?