Writers must be readers. This is what teaches us voice and proficiency of language. It also helps with those grammar skills, especially if you chose something that challenges you. You have to read things that are a tad laborious, but nonetheless rewarding. These are the same skills we use in learning a foreign language. And, more often than not, it does not fix our problems but it does make us more aware of them. Sometimes, it's enough to trigger a little voice that says, "this is not right."
That's why we cannot rely on computer spell check and even worse, computer grammar check. I recently completed the first half of a client manuscript where my very talented client knows that she is a very abysmal speller. She also knows she cannot trust spell check.
Her fantasy manuscript is riddled with errors like being executed a "trader" for dealing in "fowl" magic. Yep, can't barter for chickens in this universe.
And today, "he is quiet cleaver, though none here knows how cleaver."
Ah, the work of an editor...