Discordant fragments of lost memories rushed through the back of her mind: a poignant scene from nowhere— a girl crying for help, her hands firmly wrapped around the head of another girl who was lying motionlessly on her lap.

"Motionlessly" is okay, but "motionless" is more succinct.

The poor girl was yelling desperately for help—: "Please.

I don't think the em dash is used in this way. Before a piece of dialogue, we can use a colon.

It was strange —she had no idea where these unknown memories came from, but the sensations were too real to be just a hallucination.

Suddenly, she sensed an unsettling energy at the backbehind her, prompting her to turn swiftly around.

There, her eyes met athe silhouette of a young girl, standing stiffly near the crumbling wall, her ginger hair billowing in the wind, its striking red hue was the same as Jade’s.

"A silhouette" is okay, but I think "the silhouette" is more appropriate here. "A" places more emphasis on the silhouette, while "the" places more emphasis on the young girl.

She was looking at her, but with the jubilant smile of a child.

"But" might not be too appropriate here, since I don't see any contrasting ideas.

Then she seemed to whisper, "Nice to meet you again, sister,” andbefore she dissipated right before her eyes like mist.

"And" is okay, but allow me to suggest "before", which sounds more fluent.

Jade's eyes widened, and flashes of distressing scenes flickered in her head again, but this time more vividly, with brutmore force.

(1) It already seemed pretty vivid before, when you wrote "the sensations were too real to be just a hallucination", so I'd suggest the addition of "more".
(2) "Brute force" refers to physical strength, and isn't used in such a context.

The pain felt so raw and searing that it became hard to breathe, like someone was choking her neck.

Alternatively: "like someone was strangling her neck."

Promptly, she fell onto the ground, grittidesperately fighting against the agony, and tried to grasp for air.

"Grittily" has the positive connotation of courage and resolve. I don't think it's very appropriate here, given the context.

All her scattered senses made her feel like she was trapped in a nightmarish dream.

Of course, instead of saying "nightmarish dream", you could also just say "nightmare". I think both are fine.

Just a rRandom hHeart-wWrenching sScene from mMy iImagination

Discordant fragments of lost memories rushed through the back of her mind: a poignant scene from nowhere— a. A girl crying for help, her hands firmly wrapped around the head of another girl who was lying motionlessly on her lap.

The poor girl was yelling desperately for help—. "Please.

Help us, please…” Sshe yelled u. Until her voice cracked and she ended up coughing as she choked, hot tears streaming down her face.

The girl in her arms was completely frozen in timestill.

She showed no sign of breathing or life, sticky blood trickling down her messy red hair.

It was strange —s. She had no idea where these unknown memories came from, but the sensations were too real to be just a hallucination.

But all these puzzling things occurring repeatedly in her head appeared to be indicating something: a. A haunting past?

The traumatic memories that were lost infrom her childhood?

Suddenly, she sensed an unsettling energy at the back,behind her prompting her to turn swiftly.

There, her eyes met athe silhouette of a young girl, standing stiffly near the crumbling wall, her ginger hair billowing in the wind, i. Its striking red hue was the same as Jade’s.

She was looking at her, but with the jubilant smile of a child.

Not sure jubilant is correct here it suggests you are celebrating something.

Then she seemed to whisper, "Nice to meet you again, sister,.” aAnd she dissipated right before her eyes, like mist.

The pain felt so raw and searing that it became hard to breathe, like someone was choking her neck.

Promptly, she fell onto the ground, grittimly fighting against the agony, and tried to grasp for air.