Learning Kanji looks so hard.

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Almost perfect! Some of my corrections are just nitpicks but I'll explain.

"I'm going to learn Chinese from this Sunday. So I bought...."

We don't normally start a sentence with "so" when explaining the reason for something. You should use it in a compound sentence instead.

If you do want to write it as two sentences though you could say:

"I'm going to learn Chinese from this Sunday. Because of this I bought..."

"I bought a textbook that says how to write Kanji."

Small correction on text book - textbook should be one word.

But this sentence sounds like the textbook just says the words "how to write Kanji" on it. If you want to say that the book tells you how to do something, "teaches how to write Kanji" would be the sentence to use.

Also I changed "which" to "that" - This is just a nitpick, saying "which" here is perfectly fine, but saying "that" instead sounds a lot more natural.

"I thought that learning English is not easy for me..."

In this sentence you don't have to say "that". "I thought that learning English..." makes perfect sense, but "I thought learning English..." sounds a lot more natural.

Also because "thought" is a past tense verb, "is not easy for me" should be past tense as well. But I changed it to "hard" because it sounds a lot more natural. "I thought that was hard, but this is even harder" is something I'd hear people say a lot, but I've never heard anyone say "I thought that wasn't easy, but this is even harder".

"but Chinese is more difficult because..."

I changed that to "even more" because it sounds a lot more natural.

I'm struggling to learn Japanese Kanji right now though so I know how you feel!　お互い、頑張りましょうね！