|I'd like to begin by pointing out that I am in no way a pro at this stuff. I simply express my opinions and hope that they are helpful to you as a fellow writer!
Overall I think that this is a very effective and suspenseful story. You definitely have an intriguing plot line here, and I was hooked from start to finish. Too often I find myself losing interest when reading, for a number of reasons, from the author being overly-descriptive to the characters simply being too bland, but this story struck a very nice balance!
You manage to convey feeling here quite nicely in the language you use. I could feel the character's panic, particularly through the language used in the first paragraph. I found the words "sharp" and "flitted" especially effective. "Snapping" would also have been a great word to emphasise this feeling of panic in another context, but to me it seemed an inappropriate word to describe the shutting of a door.
One of my favourite lines in this story is "...shadows forming two broken legs, pale white arms covered in wounds that hooked onto each stair and pulled the rest of its twisted shadow-body up, and eyes that were seemingly frozen in different directions"; conjures up some super-creepy images! You certainly have a way with words, particularly when it comes to describing these creepy beings. I applaud you for that!
As regards being able to relate to your story, there was actually a time in my life when I managed to convince myself that I was seeing ghosts as a result of talking to my friends about the paranormal and the avid interest I took in it. I think it happens to a lot of people, and I think it definitely renders your protagonist relatable. Gives her a very human feel, silly as it may sound!
When I began reading this, I wasn't fond of the idea of the character writing in the notebook. I thought the jump from first to third person in the fourth paragraph was too unexpected. However, towards the end, I changed my mind. The unexpectedness actually works to your advantage in terms of suspense here. The reader is lulled in to a false sense of security during the notebook entries as all of this is in the past, but as soon as the narration switched back to third person, I was instantly reminded of the immediate danger the character was in. The only change I personally think is necessary here is to italicise the notebook parts. I think this is needed simply because it is easy to forget that the narration has changed from third to first.
I'm not sure if this would also be classed as a structural, but I thought it'd be worth mentioning here. Because of the structure of the sentence, the phrase "contrasting with the ashen colour of her skin" appears to be in relation to the notion that her hair is messy and tangled, as opposed to the colour. I like the idea of the contrast, but maybe restructuring the sentence would clarify what you are talking about? For example, "Her black hair, which contrasted with the ashen colour of her skin, split out behind her. It had become messy and tangled". On a similar note, I don't know if this was intentional, but I like that you kind of have a theme of contrast going on. It happens again in the line "...the red splatter of her own blood on the white carpet", and I find it very effective! I'm a fan of themes in stories, particularly short ones like this; they have a way of tying the whole thing together as well as providing a very visual impact.
As a final note, you've definitely managed to maintain a sense of eeriness throughout this story. I found it satisfyingly descriptive (to the point where I actually ended up with a pretty terrifying image in my mind!) and, coming from a fan of horror, it was a real pleasure to read!
Keep at it!