After several attempts I still can’t figure out how to finally cast that spell!

Grammatically fine, but I might phrase it something like "Despite all my efforts, I still can't figure out how to cast that spell!"

Well, it goes deepway back into my past: when I was younger, angrier, and desperately attempting to pull myoff a plan off.

"Deep" feels a bit too poetic here for a conversation.

Also, I've said "a plan" rather than "my plan", because we appear to be introducing one which hasn't been talked about before.

I was stepping into the deep, deep streets of the city, which called Decadence; t. The name of the city spoke to itself,: it was a desolate, decaying city, no matdespiter how polished it looked on the surface.

I came across onea strange pony, then I followed him to something that looked like a circle which embossed out ofn the road.

I came afterkept following him, and then I was shocked, because I saw a big fair!

The rest of this is fine, but I might phrase it something like, "I kept following him, but what I saw next shocked me: there was a big fair!"

Thousands of lights haung from the ceiling, hundreds of houses and tents, and the ponies which spendingspent hours glstaring from one tent to anotherat each tent.

"Hung" is the past tense of "hang".

"Glare" has a sinister connotation. A glare is scary, and full of hatred and evil intentions. I don't think this is what you mean here.

There was a metal ladder with steps, o. Of course there were precarious ones, some of the steps were broken, but in overall, the ladder was in good conditions.

Rephrased it a little to make more sense.