However, the mistake I made was in not taking my reader's needs and expectations into account. I've heard writers say that they must write for themselves, that they are their own critics, and that if they don't like their writing, how can they expect anyone else to? Although I agree with most of that, the part I have come to accept is that your reader determines how and what you write. Always.
Ignoring your audience is also an indication that you've got yourself in that "isolated writer" mentality which is an illusion and a trap. If you want to be read, you need to know what your reader wants to read. Not that you have to pander to their taste, but that you do need to be able to listen when they offer constructive criticism. This was the biggest hurdle for me, because I had to be able to trust people who judged my writing enough to listen when they told me it needed certain kinds of revision.
Ultimately, writing for your audience is a mark of professionalism, it seems to me. Writing for yourself is fun, but unless you want to please an audience of one, it won't matter as much what you think, as what the people you show it to think. The other "ultimately" about writing is that, as Plato feared, once it's been published, it's no longer in the author's hands to control what others think of it. The text is no longer all yours; you've shared it and opened it up to censure and praise.