Many people suspect that it's physical abuse, but I am here to prove you wrong. If you aren't already aware, the game has given hints that Natsuki suffers from an abusive relationship with her father. During act three, Monika gives a piece of dialogue confirming that Yuri was, indeed, a yandere. She has a hobby of cutting herself even when at school. It's implied that, like Monika, Yuri suffers from Borderline Personality Disorder (BPD for short). However, if the player actively chooses to pursue Sayori's route she will experience guilt anyway since she doesn't believe that she deserves to be loved and will thus, either way kill herself, no matter what the player chooses. Sayori is also introduced as the protagonist's best friend. It reaches a point where, after she confesses her love to the protagonist, she stabs herself to death out of over-excitement or rejection (depending on the protagonist's response to her confession, both lead to the same outcome, courtesy of Monika). She is ultimately unable to control her obsessive outbursts.
This heavily points to her identity as Libitina. In game, there is a secret poem you can unlock. Yuri displays several of these symptoms in the game, such as eye twitching, misplaced laughter and self-harm. Yuri is 18, and falls into Libitina's age range, making it possible that Yuri is Libitina.
