Grammar: What does it really take to be "very good" at it? [closed]

The most obvious mistake in your question is equating correct English with native speakers' English. There have been many discussions here about what constitutes "native speakers' English", largely inconclusive: eg What makes a non-native speaker sound foreign? . The answer to that question is probably "a host of small things, most of which are beneath the threshold of perception." The answer to "What constitutes correct English grammar?" is easier to get an answer to: whoever you are speaking to, it boils down to "The way I myself speak and write English." (Example in the last sentence: whomever is technically correct by the rules you refer to, but probably has not been used by any native speaker for some years save for effect. Do you really want to use it?)

At present your English is good enough not to draw attention to itself, which is the only important criterion. Beyond that, it really is a matter of style and experience.

You are at a level where you will do better by trying to let go of your worries, stop studying grammar directly, except to the extent that you love doing so over reading or studying other things, and read for its own sake.

You will almost certainly never learn all the grammar rules that a moderately highly literate native speaker has learned incidentally. But the idea that it will embarrass you is irrational or maladaptive. Most editors will be very happy that you have some errors so they can feel value for doing their work!