Formal letter writing can be a bit embarrassing especially for someone who is coming from an “sms” kind of background where every word must be chopped. But it never means that one is not smart enough. There are many free resources that can you can use to train and perfect yourself in the art of formal letter writing.
First, there are many great books that you can buy or even borrow and learn the basics of letter writing, for example The Elements of Style by E. B. White and On Writing by William Zinsser. If you happen to be a person who does not love books, then you can use the internet to get all the information free of charge, and most of that material is collated and extracted from some of the classic books.
These online and offline resources will orient and cement you in all the basics and principles of formal letter writing. I was once just like you now, and I kept working on improving my writing skills through endless practice and references to whatever resources that I could find. Over the years, matters such as audience, purpose of writing, types of formal letters, salutations, paragraphs, tone, structure and much more finally became clarified to me. So have faith and keep going.