I write well. I've had many english classes in my life, including four college-level english classes. I did not learn to correctly identify all the parts of speech until I had to in order to learn a second language. It was really tough in the beginning, because foreign language professors tend to take for granted that you know english correctly (which I did, intuitively, but I couldn't articulate the rules or apply labels to adverbs and articles and prepositions, etc.).
Long story short, your educational system has failed you. They do not emphasize the importance of grammar and writing nearly enough in school.