Someone told me that you should always think in English in your head to improve your English so I tried it. Someone told me that you should always think in English in your head to improve your English so I tried it.

Looking at the Japanese version: いつも英語で考えると良いと言っていた人がいたので、やってみた "so" works the same way as ので here and it flows much better as one sentence.

Someone told me that you should always think in English in your head in order to improve your English. Someone told me that you should always think in English in your head in order to improve your English.

Someone told me that you should always think in English in your headorder to improve your English. Someone told me that you should always think in English in order to improve your English.

"in your head" sounds a little redundant, but the grammar is still correct.

Then I tried it.

ThenSo, I tried it. So, I tried it.

ThenSo I tried it. So I tried it.

IBut it was difficult, I realized that due to my lack of vocabulary. But it was difficult due to my lack of vocabulary.

I can't tell if "that" was technically wrong here... but it feels a bit wrong. Firstly, let's think of "that" as a word that conjoins to sentences into one, the second describing the thing marked by "that". Well, "my lack of vocabulary" feels not like an independent statement, but the start of a new sentence like "my lack of vocabulary makes it difficult."

It was difficult, as I realized that myI lack ofed sufficient vocabulary. It was difficult as I realized that I lacked sufficient vocabulary.

It was difficult, and I realized that my lack of vocabulary. It was difficult, and I realized my lack of vocabulary.

It was difficult, because I realized that my lack of vocabulary. It was difficult because I realized my lack of vocabulary.