Do not worry. Do not try too hard to use "advanced" words. Simple writing is good writing.
George Orwell was a master of prose style. In his essay, "Politics and the English Language," he says "Never use a long word where a short one will do."
Use basic vocabulary whenever it expresses your meaning.
Only use a fancy word when it is useful. Only use it when it means something a little different from the plain word, and you know for sure what the difference is, and it expresses your meaning better than the simple word does.
Your vocabulary will expand naturally with reading. Don't force it. Just keep reading, and look up the words you don't know.
Here is a passage from one of the most famous speeches of all time, given by Winston Churchill during World War II. He was a highly educated man and the author of many books. This is good English; in fact, this is great English:
"We shall go on to the end, we shall fight in France, we shall fight on the seas and oceans, we shall fight with growing confidence and growing strength in the air, we shall defend our Island, whatever the cost may be, we shall fight on the beaches, we shall fight on the landing grounds, we shall fight in the fields and in the streets, we shall fight in the hills; we shall never surrender."
By my count that is 75 words, of which 66 are words of one syllable. And notice what Churchill does not do: he does not use synonyms to avoid repetition.