I’m still living the same unchanging days,Each day is much the same, and my [subject noun phrase] is showing no signs of progress.

Living cannot take the object "days".

"Showing no sign of progress" - there are two issues here. First, it must be "signs" and not "sign" (this is a fixed expression). Second, it doesn't sound right without a subject noun phrase attached. If you wanted to say it without specifying what is progressing, you could say "there has been no progress to speak of" or "there have been no signs of progress."

I grow weary of'm frustrated by my own lack of talent.

It is not typical to say that you "grow weary" of something you're the cause of. However, I do find the sentence as you wrote it thought-provoking. It makes the reader think about how one can feel powerless in the face of one's own traits. I think it would be a great sentence to keep in, I'm just adding the correction as an FYI.

My drawings remain clumsy, and I string together one poor draft after another, only to disappointing myself again and again.

I [x] only to [y] implies that [y] happened entirely after [x]. Since each draft causes the protagonist to be disappointing anew, you need to use the progressive aspect.

Even after my parents supported me through high school, I didn’t go on to college or find a job.

In America, it would not be normal to say "my parents supported me through high school" because this is viewed as an obligation of all parents to their children (however, it would be completely normal to say "my parents supported me through college." This would probably also be the case in England/Ireland/Australia/New Zealand/South Africa.

If you are writing for people from high-income English-speaking countries, it might make more sense to say "Even after all my parents did for me in my youth", to emphasize/clarify that the character feels indebted to their parents.

Instead, I’'ve been made them watch asing this foolish dream as their only son their only son chases a foolish dream.

Putting "as their only son" implies that being their only son is the foolish dream.

Some draw astonishingly well, and some create works I could never imagine, even if I stood on my head in a million years.

The idiom "even if I stood on my head" is sort of comical/lighthearted, and doesn't feel right in this context.

I'm sorryALSO POSSIBLE（アメリカ式スペルで ) : I apologize for themy delay ined reply once again.¶
(イギリス式スペルで ) I apologise for my delayed replying once again.

NOTE: The original is possible.

BETTER: I tried many times to write to you, but each time I picked up the pen, I couldn’t find the words.somehow the right words did not come.

NOTE: The original repeats the word "I" too much. Also, many people today with a computer -- not a pen.

I’m still living in the same unchanging days, showing nocircumstances, with no clear sign of progress.

ALSO POSSIBLE: How long mustshould I keep crawling forward like this?

No matter how hard I try, all I can come up with are tired, ordinary stories.

My drawings remain clumsy, and I string together one poor draft after another, only to disappoint myself again and again.

ALSO POSSIBLE: Why did I ever think I wantedaspire to become a manga artist?

It’sALSO POSSIBLE: I feel as if I’ve repaid their kindness with disgrace.

Seeing someone like me—, it’s only natural that you’d be feel disappointed.

LivingMY GUESS: After you leave in theis city, you’ll surlikely forget all about me inover time.

They have real talentオプション：この文は冗長です。削除しても問題ありません。/ OPTION: This sentence is redundant. It could be safely deleted.

I’m painfully aware that it’s people like them who become professional manga artists.

MaybeWITH A SOFTER TONE: Sometimes I feel as I’m just wasting my life.

Takeshi Kitahara