Revision of Crappy First Draft

It took me several weeks and I stopped counting how many drafts to come up with the first revision of my crappy first draft. (See previous post.) I finally realized that even though I put all that effort into it and lengthened it from six pages to thirty-three, it is still just that: a revision, a slightly less crappy second draft. It was not encouraging to me when the teacher said that sometimes you have to write something forty times to get one good paragraph. I figure if I have to do that, I don't have much of an idea of what I want to write about or how I want to say it. Maybe instead of putting all that time and energy into one paragraph, I ought to start over completely. I know what my teacher was saying and I don't really disagree with her, but I hope she doesn't mean that I have to write that much for every good paragraph.
Sometimes I end up with a paragraph that is so good it stands by itself and then what do I do with it? Almost every revision has something worth saving, but what do I do when I end up with parts worth saving from several revisions and they don't fit together?
Another issue: how about length in non-fiction, essays, etc.? My teacher also said that most journals want essays that are around 15-25 pages long. I've always written short essays so that figure seems daunting. What is really daunting about it is that you have to be a helluva writer to keep someone reading for 15-25 pages. But what choice do I have? I have to keep trying. Quantity and quality.