So the story is called The Tallmare and it's a horror fic I started. As you can guess the main monster is my take on the slenderman or this case the tallmare which is recontextualized to fit the mlp universe.

The story

Now what I think I struggle with is describing things and dialog.

The dialog: why I think I'm struggling with it is because it makes seems out of character. Like there's a part where It seems forced and just like something that the characters wouldn't say.

Descriptive writing: keep in mind sometimes my autism makes it hard for me to think of words that describe things .and thus i use more simpler words. This can be slightly fixed and its mostly like when describing emotions.

I think you need to give a few examples of the dialogue. We can't really know how to improve it if we only know that there's an issue. The best answer I can give with the information here though is that you should put characters in scenarios where the dialogue is natural. If you have important exposition that needs to be said, you should have a setting where that exposition could potentially come up. If you're just trying to find ways to make your characters talk to each other, you need to have your characters have chemistry. They don't need to talk about exposition. Just anything that moves the plot towards another point. Another thing that could help is for you to just stop and really dissect who your character is. You can just brainstorm for a page about anything the character is in to, different things of the character's past that might've shaped them into who they are today, or just regular things like what their favorite show might be or their favorite food is. This might help you get a grasp on who your character is and when you try to write their dialogue, you can tell more easily what is something they would naturally say. Do this for both characters involved in an interaction and it should help you prevent writing scenes that are out of character.