Another self-deprecator reporting in.
I know that I can write well based on all the feedback I've got, at least in Finnish, but damn it if I'm ever happy with anything I manage to blurt out. And that's what I do if I'm required to (or make myself) write something, I blurt it out, because I always wait for the last possible moment to do it or sometimes even beyond, convincing myself that the more I think about it beforehand, the happier I'll be with it in the end.
The only situations where I'm at least partially happy of some writing I've done is when I come up with an original idea or a gimmick in the text, but in those cases I usually hate the way I wrote it.
This bothered me quite much when I started posting on forums as every post I wrote looked like it was produced by an idiot and every single one didn't have enough point or length to justify it's existence. What helped me stop being overly critical of my posts was taking part in Sean's RTD (which I didn't expect to become so incredibly big) as playing that game I was "forced" to make short posts with little substance (though I still don't think I have any posts there that consist of one action and nothing else)
So, I join all the people who say that you just have to write more, it really should help.
Also, when playing an instrument, there's always so much to improve that it's not even funny, especially when performing to people, no matter what they say.
However, when I do something that I don't consider myself good at, I can be immensely happy with the outcome.
P.S. I still use quite large quantities of time to make my longer posts "perfecter" and almost always feel that they're inadequate, like I do with this one right here.
EDIT: Also, I use the edit-button more than is healthy, a terrifyingly large number of my posts have faced some "enhancements" few minutes after posting them.