Your sentence effectively sets a melancholic and suspenseful tone, drawing the reader into Alyssa's emotional turmoil. Here are a few strengths and areas for improvement:
Strengths:

Strong Imagery & Atmosphere – The rain, thunder, and darkness create a vivid and immersive setting that mirrors Alyssa's emotions.
Emotional Depth – The mention of guilt, regret, and fear makes Alyssa's experience feel raw and relatable.
Engaging Hook – The opening immediately captures attention and raises curiosity about what happened that summer night.

Suggestions for Improvement:


Tighter Sentence Structure – Some phrases could be streamlined for better flow. For example:

"The rain had been a fitting backdrop to the darkness that enveloped her that night." → "The rain was a fitting backdrop to the darkness that consumed her."
This avoids repetition of "that night" and makes the sentence more concise.



Show, Don’t Tell – Instead of stating that "Alyssa found herself standing by the edge of the forest, her heart heavy with guilt and regret," you could subtly show her emotions through her body language or thoughts. For example:

"Alyssa stood at the forest’s edge, her breath uneven, the weight of guilt pressing against her chest."



Vary Sentence Structure – The sentence "Alyssa's hands were trembling as she clutched the phone, her green eyes reflecting the fear that gripped her soul." could be restructured to avoid too many clauses in a row:

"Alyssa clutched the phone, her trembling hands barely able to hold on. Fear flickered in her green eyes, mirroring the storm inside her."



Overall, this is a strong, atmospheric passage that effectively conveys emotion and suspense. With slight refinements, it could be even more immersive and powerful.