What it can it be?

Might be better to say "You may be wondering [what can it be]?". Without the earlier part, it sounds like you don't know yet.

You canmay think it’s strange, but I think it’s The Little Prince.

May expresses possibility and uncertainty better than "can" in this situation. Because this is an opinion, use "I think" or "I feel"

The matterthing is that The Little Prince isn’t a children book.

"matter" in this statement is too formal. "The thing is..." is a conversational, informal phrase, so using "thing" instead matches the tone better. Not wrong, just sounds strange.

Once, when I was losing my motivation to learn English, I wanted to find ‘an easy reading’ and I recalled about thethis book.

"this" shows it is The Little Prince book in particular that you recalled.

Ugh… I wasn’t depressed for a long fortunatelytime...

You seem to want to focus on the change, that being how your mood has changed for the worse. "Fortunately" does not really flow with that idea because it describes what was.

I found out that the author’s mother called him ‘Little King’ and then I supposed that Little Prince dihadn’'t died.

"did not" is the negative past tense which would modify "to die". Conjugating both makes it sound weird. If you want to use the negative past perfect tense instead, you can say "had not died" or "hadn't died". Past perfect tense sounds more sophisticated and fluent but both tenses work!

He and Aviator (and the author probably) aren’t separate personsdifferent people.

"Separate" is more physical, while "different" is more general and broad. People are already separate because they have different bodies. People are different and have different personalities though.

THowever, that time I washad watched the 2015 adaptation of the book (2015) and, it seemed to me that the message behind the cartoon’s story is (it's still is)was exactly the opposite - don’t greow up, don’t let go of your little prince, don’t become an aviator.

To emphasize how opposite of your interpretation it was, use "however" or some similar conjunction. "2015" in this instance is an adjective, so you can put it before the noun.

The Most Depressiveng Book I’ve Read

Depressive is a rather niche, medical word, mostly encountered in the compound term "manic depressive" which applies to the person experiencing the emotions

Depressing is the more usual word for something that makes people feel depressed

What it cancould it be?

You canmay think it’s strange, but it’s The Little Prince.

"can think" implies no opinion as to what you think the reader thinks of your opinion, but if that was the case, why comment on it? Because of that "you may think it's" is much more common.

The matterproblem is that The Little Prince isn’t a children's book.

So, they expellremoved a few characters.

"expelled" is more niche in its usage, and is more used in the context of removing a person from an institution (e.g. a school or society)

Of course, tTher Snake wasn’'t Tthe Snakre.

The little Prince just left away for the stars.

"left" already contains the implication of going "away" from somewhere, so it sounds weird to say "left away"

Once, when I was losing my motivation to learn English, I wanted to find ‘an easy reading’ and I recalled aboutmembered theis book.

"recalled <something> about this book" would be used, but then "something" would have to be a specific fact about the book.

If you just remembered the book existed, "remembered" is simpler and more natural.

Ugh… I wasn’t depressed for a long(long / a long time), fortunately.

"long" is an adjective and doesn't take "a", unless you pair with a noun like "time" (and then it's really the noun taking "a")

I found out that the author’s mother called him ‘Little King’ and then I supposed that Little Prince didn’t died.

ThatOne time I was watched the adaptation of the book (2015) and it seemed to me that the message behind the cartoon’s story is (it'sand still is) exactly the opposite - don’t greow up, don’t let go of your little prince, don’t become an aviator.

"that time" would imply you had already mentioned the time you watched the cartoon, but since it's the first time bringing it up, "One time" is better.

"I was watched" -> someone looked at you
"I watched" -> you observed something else