Can anyone help me to understand what my English teacher means in this advice he wrote at the end of my essay?

Your teacher is suggesting that you instill some connection between the big reveal and what leads up to it into your story. Anyone can write a story about a murder and have the murderer turn out to be some random character, but a good writer will have the ending connect to the narrative. It won't be a twist just for the sake of being a surprise that the reader wouldn't see coming. Once the reader learns the murderer's identity, he or she ought to be thinking "Aah, I should have known. If I'd looked at the clues I would have guessed long ago." That's why writing a good murder mystery story is much more difficult than people assume it to be. Without clues that foreshadow the reveal all you're really doing is attempting to throw everyone for a loop. And if you do that, you'll need to explain the motive cold, out of nowhere, and seeing as it's the end of the story, readers won't be keen to slog through that. A sloppy writer just throws anything onto the page. A good writer crafts a tale.

I'd advise you to ask your teacher - but I'd take it to mean that something earlier in the story should give a tiny hint - a suggestion - of her guilt. It would need to be very subtle but it could be extremely effective.

Writers often use imagery to help tell their stories. The image of a mirror would raise ideas about the woman seeing herself, or catching a glimpse of someone else in the mirror. Sometimes mirrors reflect an image that isn't clear, either because the glass is foggy, dusty, etc., or because it's not straight. Any of these things can cause an image to be distorted. You could think about how she sees herself and how the mirror image can express that in the story. Or how she sees the others. You could also play with less realistic ideas, like her catching a glimpse of the mirror and thinking it's something else, like a ghoul or something, and then looking again and seeing that it's herself. What does this tell her (and the reader) about herself? These metaphors are fun to play with and can say a lot about character without the writer having to explain it all. "Show, don't tell", as they say.

Yes. All you have to do is ask him what the hell he meant. Hope this helped.

You should have asked him to explain what he meant. That's what he's there for and we don't always understand what people mean when they speak. Just ask him to clarify.
It sounds like you didn't make the twist clear enough for the reader to know the mother was her daughters killer and to put a real punch to the twist the teacher wants you to elaborate the moms thoughts and reflection of the murder itself. It is a good story.

the psychopath murderer hating herself, and lashing-out at others with the pain and anger she feels for herself.
that is internal anger being repressed, and expressed as violence to others through the psychological mechanisms of denial, deflections and transference; thus she is "blaming others" for her own suffering and misery.

check this article on psychology's "dark triad".

By asking your teacher will show him you care about improving your work and give you a better grade.......it sounds to me he needs more details...like the mom was a bully and the daughters best friend knew this because friends tell their best friend everything....but. If you are wanting to hide the murderer until the end.(which is really clever....cause it leaves the reader going..."oh man,I didn't see that coming)....then let the teacher know this and give you example what he would use as a reflection.

I am thinking that he just means to have more mirrors and that as descriptive scenery for it all. a sort of tip off to the reader for it. If you think that it stands well all on its own, then leave it.