In theThis morning, I decided I wouldn't write today.

"In the morning" can be used as a generalization for any/all mornings, or for the following morning. Examples: I have to get up early in the morning (this means tomorrow), and The rooster wakes up early in the morning (this means every morning).
To speak about today, it's "this morning."

This is describing a decision you made in the past; you're telling us what you thought or decided at that time, not now, so it's "wouldn't"

Not because I doidn't want to, but because I caouldn't find any topic to write about.

Keeping the same tense as above

So if you read/are reading this, you aren't reading it.

"Are reading" is a better option because it is what the reader is actively doing at this moment. But what you wrote is not incorrect either.

I started writing about how I'm no wasn't writing, which technically counts as writing, so now I'm stuck in an existential paradox.

I stared at the ceiling for 23.23 minutes, but all I got was a spider in the corner.

This is getting off-topic, but 23% of one minute is 13.8 seconds (this is what 0.23 minutes is). Probably better to write 23 minutes and 23 seconds if that's what you meant.

I think she writes more than I do today.

Is her name Charlotte? :)
Just a reference to a story about a spider who writes in her web, in case you haven't heard of that.

If a text is not exists(doesn't exist / is non-existent), it shouldn't be perfect, right?

Anyway, thanks for not reading this non-existent text, I appreciate non-attention.