Grammar test and grammar principles scare a number of people out of their wits. However, it must be admitted that grammar is undeniably, an essential part of writing. Without grammar, language loses its sense, to be sure.
There are people who have inbuilt skills or talent. They do not have to learn anything either in theory or in practice. These are far and few in between. The author has pointed out this face about his own self in a frank manner.
"My language skills come from a life-long love of writing and reading. Not some high-falutin' college courses.'
I know of singers, poets and painters who had no formal training in their respective arts. Yet, they excel in their field. This is a gift which is not within everyone's reach.

Write on!