Oaiko.

High school student Kiyoharu has a cute girlfriend and is at the top of the caste system. However, he feels suffocated by the fact that he has to hide himself and suppress his true feelings. Then he becomes friends with Mizuki, a member of the library board, because of a secret they share. With Mizuki, he's able to reveal his true self, and the two become closer and closer. Gradually, he begins to prioritize his time with Mizuki over his girlfriend... (Source: Core Magazine, translated)