I think many people--like you--can learn general grammar rules by reading. I am also one of those people, BUT I have always felt insecure in my writing. There are many fine points that I did not absorb and that I make mistakes in. I have also noticed that my daughter, who reads no more than I did at her age but who is made to do a rigorous grammar course, can read at a higher level than I could at her age.Reading Jean's comment, I'm thinking ... I probably would have been insecure about the fine points of grammar IF I had ever thought about the fine points, or cared. But I didn't! I didn't think about grammar at all, I thought about writing (comma splice intentional).*
I write by ear. I don't recall ever consulting a grammar book, not once in an entire career of professional writing. In fact, I didn't even own a grammar book until a little over 10 years ago, when an editor told me that all the editors in New York liked The Grammar Bible. I bought it, but I didn't read it. (Hope to do so one of these days.)
I always wanted to be a writer, from the time I knew what writing was, and I was an obsessive reader (and still am). And I simply never gave grammar a second thought. I learned grammar through reading, and I practiced grammar through writing.
I wish now I had been taught grammar -- sentence diagramming in particular -- as I would have loved every minute of it, and I think formal instruction in sentence syntax would have made me a better writer sooner.
But I wasn't taught grammar, and I learned to write without thinking consciously about grammar and punctuation.
Back to the question of missing the fine points: I distinctly recall, from time to time (especially back when I discovered my affection for the semicolon) not knowing what 'the rules' said to do. But uncertainty about the rules never caused me to think I ought to actually go out and look up the rules.
Basically, I had just one ironclad rule: does it sound right? If it didn't, I rewrote; and I rewrote over and over and over again. One of these days I should count how many versions some of my sentences (and passages) go through. It has to be in the hundreds. Many hundreds, in some cases.
I'm sure that, like Jean, I was making subtle errors all those years. In fact, I know I was. After I finally started to learn the formal rules, just 2 years ago, I discovered one in particular that I hadn't picked up through reading, which is the prohibition against placing a comma between an independent clause & a dependent adverbial clause.
I went home because I felt sick.
I went home, because I felt sick.
I had never heard of this rule, and never conceived of it, either. (I had also never heard the rule about using a comma after a FANBOYS, or the rule about not putting a comma after "rule" 2 sentences ago.) Where commas were concerned, I had always followed my own rule, which was to use a comma if it sounded right. So sometimes I used a comma, and sometimes I didn't use a comma, depending.
After I learned the No Commas Before Subordinate Adverbial Clauses rule, I started to follow it .... but then, not too long afterward, I stopped. The rule doesn't work! Sometimes a sentence needs a comma, rule or no rule, and there's an end to it.
All of this said, I feel pretty strongly today that I would have been better off if I had learned formal grammar, including sentence diagramming, in K-12. But that is a subject for another post.
* The fact that writing is grammar, pretty much, escaped my notice.