She tried to calm herself down, but it didn’t end wellwork.

"It didn't end well" sounds like something really bad happened because of her attempt.

She wondered, if there’s someanypony who struggleds as much as she is right now.

Her hooves touchrampled the grass under her, and no matter how she tried –, she couldn’t stop thinking about it, about Celestia’s task; she noticed how sunlight filtered through branches and leaves, wrapping her in warmth.

I changed "touched" because it makes it sound like she's just poking the grass, or floating over it and jusr brushing against it. Maybe she's flying? But if she's walking, I suggested an alternate word, which means that she's crushing the grass. You can use other options too.

Sunset’s mane symbolized her tendency to made fast decisions and sometimes be mad at someonehot-tempered; the sun emitted warmth, just like Sunset, but in other way.s. (or "another way")

"Hot-tempered" means she tends to get angry easily. You can also say "hot-headed".

She wandered through the park, but still, she couldn’t do it,: she couldn’t find athe courage to make a friend.

I put a colon (:) here because it's used to introduce something you were just talking about. It gives more emphasis.

How it’ss it possible to be failed in such an easy task?

When a sentence becomes a question, the word order changes around.
"That how it is." -> "How is it that...?"
Because the word order swapped, you can't write "How it's possible?" but must write "How is it possible?" or "How's it possible?" This is a common stumbling point for English learners :)