You sound a bit like me and visual art: I've got too much of a critical eye not to see the flaws in my cack-handed attempts to draw things, and so it's very easy for me to get discouraged. I'm a little glad I wrote many of my own million words of crap before I recognized their shortcomings; my critical faculty and my skill developed in tandem, there, so I was always having fun and rarely getting discouraged.

The most useful tip I can think of to give you is, write other things, too. If you spend years on the same story, going over it again and again and rebuilding it and then fixing things and then changing stuff again . . . I did that for about four years -- until the same age you are, actually -- and then finally realized that, while there were things about the story and characters I very much liked, I kept partially tearing down what I'd built and then slapping more on top, with structurally unsound results. So I laid that project aside, with the intent of letting it fade in my mind so that when I came back to it, I'd have the perspective necessary to remove the stuff that wasn't so great and refine the bits that were. I still periodically go back and knock the dust off it, to see where it stands. But in the meantime, I started working on three or four other things -- new ones, that had nothing to do with the older project -- and by stretching myself out in different directions, I developed my craft muscles much more effectively.

In other words, multiple projects are probably good for you. The lessons from one will help you with another. If you keep focusing on one thing for too long, it's easy to lose perspective on it, and not know whether it needs major revision or is ready to be sent out.