My reply might contain a shit ton of spoilers but that's the only way to help you.
In this game, there are 4 girls as you know:
Sayori, Natsuki, Yuri and Monika.
Monika becomes self aware as the game progresses and begins to alter the other girl's stats to make you, the main character, avert your eyes to her... yeah, to 'Just Monika'. This explains why she is always so calm and cheerful and as the story progresses the other girls begin acting out of place and tend to harm themselves. Whenever you'll neglect one specific girl out the 4 except Monika, they will eventually become sentient and at the end, kill themselves... As Monika is controlling every fucking thing, your choices that you make have literally the 'least' effect on the girls. As a character dies in the game, Monika, now the mastermind of the game, deletes their character files which means they are no longer existing, and that is the way the story progresses... There's no need to save anyone. Sayori will die and so will the others, until Monika is the only one left. That's the way it's meant to be played. There's simply no point in trying to save Sayori or anyone. The player needs to tweak with the character files (the .chr files in the DDLC directory) and at the end when Monika is the last one alive, you have to delete her character file to see the true ending of the game which contains references to a future game coming out later this year in 2018. You might as well see random files or pictures appearing in your game files as you progress through the game... search online to find tons of scary eastereggs the game contains.
Sato Kun