"The scent of roses filled my tiny bedroom as I rise up from bed on a beautiful morning." You ROSE up from bed because the story should be in past tense. This issue appears quite a lot. "The scent of roses filled my tiny bedroom as I rose up from bed on a beautiful morning."

"I stretched and yawned, giving myself a moment to adjust my eyes for the light shining through my white, silky curtains." "I stretched and yawned, giving myself a moment to adjust my eyes to the light shining through my white, silky curtains."

"I slipped off of bed and pushed my flying curtains to the side and gazed at the golden sky." There are a lot of 'and's. You're also using a weird preposition that can be adjusted. "I slipped out of bed, pushed my flying curtains to the size and gazed at the golden sky."

"I spotted a light shadow, but as I was walking towards the door, it disappeared." Tenses. "I spotted a light shadow, but as I walked towards the door, it disappeared."

"Wake up, get dressed, work for a few hours and comeback home with all my energy drained." You're making a list. It's a dependent clause (not a complete sentence) because it's missing a subject. Also, tenses. "I woke up, got dressed, worked for a few hours then came back home with all of my energy drained."

Try to form a compound or complex sentence when you have a sentence that starts with a conjunction.

"I peeked one eye open and I see another envelope lying on my floor." "Peeked on eye open" sounds awkward. Tenses. Also, you lay a direct object down. In this case, the envelope is the direct object. Correct me if I'm wrong. "I opened one of my eyes and I saw another envelope laying on my floor."

"My mind was thinking..." You think. Not your mind. "I was thinking..."

"Now this was seriously making me curious." Tenses. "Now this seriously made me curious."

"'Eunmi! Remember you have another late shift tomorrow night!' I groaned but nodded." Technically if there is not obvious sign of who's saying the quote, then it is thought that the next person doing the next action said it, and I doubt Eunmi said that to herself. "'Eunmi! Remember you have another late shift tomorrow night!' the shop manager reminded me. I groaned but nodded."

"On the way I was walking, I got a glance into a music shop." You don't "get a glance" into a music shop and you don't need "I was walking". "On the way, I glanced into a music shop."

"...pressed my ears against the thin walls." there was probably only one wall. "pressed my ear against the thin wall."

"...until the music went quieter." Until the music quited/the volume diminished.

"The letters just keep on coming and coming." tenses and redundancy. "The letters just kept on coming."

"It became a habit of me expecting a letter each month." "Expecting a letter each month became a habit of mine."

There's actually a lot more but they're basically all the same issue: tenses. Also, there are grammatical errors in the letters, but I'll ignore those just in case they're just poetic devices. I'm kind of sleepy so I'm getting a little lazy. It's 11:00 PM here, lol.