I think one has to take into account that it takes different strokes for different folks, not to mention the circumstances that caused you to sit down and write what you are writing in the first place. How I write depends on a multitude of factors ranging from the audience, the reason I'm writing, the permanance of what I write, and the amount of time I've been alloted (or alloted myself) to write it. For instance, during an in-class essay, where I have a quarter of the time I'd ussually use to write an essay, I tend to be very methodical. I make a concrete outline of my arguement on a separate piece of paper, breaking down my arguement into its separate components, premises, and conclusions. Then I proceed to write my essay. It has come to the point where my conscious automatically fills in and decorates everything around the arguement. It's mostly manual labor, really.
Do I enjoy this kind of writing? Hardly. But it's very efficient, and it gets me my 'A'. I know that nobody except the instructor is going to read it, and so I don't have to care about how it will look tommorow, or next week, or next month. It is temporary and to the point. On the other hand, when I write something that I think is going to be read a few times over (even if only by myself), I tend to be much more careful. I'll read it over constantly, often times starting from the beginning each time I complete a sentence or paragraph, just to see if what I've added gels with the whole. I can get rather obsessive about it, and I'll constantly revise and edit it.
Another element that I think plays into how one writes is one's motivation. Are you writing because you have to, want to, need to, or most likely some combination? Are you writing to relieve stress, give energy, take energy, give attention, gain attention... all of these motives, whether you are consciously aware of them or not, play a huge role in how you write. I tend to do as you do, and simply follow my instinct first. It's only later that I'll look at what I wrote and realize the motivation behind it. I used to be much more self-conscious, and sometimes I still am. But it is more exciting to shoot first and ask questions later. It allows you to create something that comes from much deeper, and can tell you something about yourself and your inner desires and fears. It's the same with all art. When I play guitar I often completely space out and just let my fingers lead me across the recesses of my mind. But the whole effect is ruined if I think about what I'm playing. It's like dreaming: once you realize it's a dream you wake up immediately. I've heard of some people knowing it's a dream and not waking up, which does make me wonder... I don't know if I could get to that point of knowing that I'm creating and still being able to be deep and subconscious about it. But I'd assume that's the highest nirvana one can reach when it comes to art.