A couple of belated thoughts, from various angles:
1. To me, it seems like blogs/journals and writing-for-publication are (by and large) very different categories of things. They may overlap in things like memoir and newspaper columns, but (for example) writing a complete work of short fiction (or a complete nonfiction article) and sending it out to editors (even to the editor of a high school literary magazine or newspaper) seems to me to be a very different thing from writing "unmediated thoughts spat out on the page, never looking back." The former is, it seems to me, generally an attempt to join the adult world of publishing (or to prepare to do so), rather than an attempt to create the self.
2. I never kept a journal 'til my Wanderjahr (well after college), except for a brief foray into journaling in early college, and I wouldn't have shown it to anyone if I had; then, too, I would probably have said my sense of self was pretty solidly formed by high school. I think the closest I have to what you're describing is my membership in an APA, starting in early college, where my writing was indeed "self-indulgent, maudlin, self-congratulatorily clever." But in high school, I was writing fiction intended for publication (and sometimes even sending it out to magazines), and I didn't see what I was writing as being bad; I considered it as good as the sf short stories I'd been reading since I was small.
One reason for that flawed self-evaluation of my work is that I did have a talent for syntax. Teachers had been praising my writing to the skies for as long as I could remember. It may not have been 'til Clarion or later that I figured out that what the teachers meant (though they may not have known it) was "You're really good at putting sentences together," not "You're really good at the art of storytelling." But by the time I figured that out, so much of my identity was wrapped up in "being a writer" that I couldn't just put it down and step away from it. So on the one hand, someone probably should have told me (ideally gently, without using the word "sucks") that my early stories were not really all that good as stories (and that that was okay, 'cause I was just starting out); on the other hand, if they had, I most likely wouldn't have kept at it long enough to get better at all the things I wasn't so good at. (...It also helped that for our high school's Career Day one of the adults they would bring in to talk with us was a fiction writer, and each year whoever they'd brought would caution us against the folly of believing we could make a living writing fiction. So I had realistic expectations about that side of things from early on.)
3. I attended a writing-workshop meeting a couple years back at which the attendees were supposed to bring their earliest surviving fiction and read it aloud to the group. It was meant to be fun, and it kind of was, but I was surprised at how hostile some of the participants were toward their younger selves. One person read an extended section of a novel they'd written in high school, and kept interrupting their own reading with uproarious laughter about how truly awful it was. Which made me a little bit sad, because I thought it wasn't bad; I certainly see much worse in slush, from adult writers, every week. So I don't think it's necessary to be all *that* hard on one's own early writing, either at the time or later.
4. To me, it's not so much that there's this thing called "suck," a general badness that applies by default and that the good writer must continually pull themselves out of; but rather, that a given writer at any time of their life is going to be good at some aspects of the general area that we call "writing" and not as good at other aspects. I would add that some people start out good at some aspects, and some people need to work hard to learn to become good at some aspects. "Writing" is not a monolithic single thing.
For that matter, opinion about how good someone is at various aspects (and how much that matters) can vary tremendously among readers. Usually when Karen and Susan and I disagree about a submission, it's because we're focusing on different aspects of it (Karen loves the poetry of the language, for example, while I'm annoyed at what I see as the implausibility of the backstory, and Susan feels that the story is too superficial -- though any of us might take any of those stances), but sometimes we disagree even about a specific aspect. (Karen and Susan feel that the ambiguity of the ending is what makes the story brilliant, while I feel that it ruins the story. Or I feel that the narrative voice is perfect, while one of them hates it so much they can't bring themselves to read the whole story.)