A book about a girl with either a split personality or maybe possessed!

Hi, I hope this is the correct place to post this.

I was wondering if someone could help me find this book? I started reading this maybe back in 2017 or maybe 2018 but I stopped reading a little bit into the book. And I didn’t read anything for years. But in the last two years I’ve gotten back into reading and I vaguely remembered about this book but can’t remember what it was called.

So from what I remember, I think there was this girl who goes to a boarding school (maybe?) and I think she has a split personality or maybe she is possessed. And from what I remember, during the day during school hours it was the nice girl personality that did all the socialising and attending classes while in the middle of the night it’s like the evil side of this girl that comes out, and I think she keeps sneaking out of the property (I think) also from what I remember the good side of the girl and the evil side of the girl communicate together through a journal. I also think the way the book is written is as if someone is writing in a journal.

I hope this helps. Thank you :)