The Life Sentence (sentences are about life, after all)
Consider: a woman comes to you for directions to the post office. You tell her, "How about you just start driving and, when you don't arrive at your destination, I'll tell you how many wrong turns you made and grade you. Then next time you try to guess a better path, okay?" No! No way in hell would we follow that kind of direction. And yet, in our classrooms, every day, that's how kids are taught to write.
Graders receive vulnerable, beautiful, rich essays rife with metaphors and emotion and creative thought. They respond: frag, run-on, frag, frag, frag, cap, frag, new paragraph, frag, run-on. When I was teaching, we called this, "being fragged." The process is upside down.
The place to start is not with fixing; the place to start is direction. When asking for directions to the post office, you expect someone to actually tell you how to get there, right? So why do you expect yourself to know all the intricacies of a sentence if you've never been taught? If you've been fragged all your life, you may actually be limiting your expression to only a couple sentence structures. Yawn. (frag)
Since you're not in my class and I'm not teaching college anymore, I'm pointing you to super-secret spellbooks for writing sentences. Wave a "paired construction for contrast only" (also known as pattern 16a) and watch a sentence jump through hoops. Or maybe "use a dash to set off an abrupt break or interruption and to announce a long appositive or summary" and you'll end up thinking, "Wow? That's legal?"
The first reference I make is to The Art of Styling Sentences by Ann Longknife and K.D. Sullivan. The second is a quote from The Elements of Style by William Strunk Jr. and E.B. White. If I were teaching you about sentence construction, we would go through example after example and we would practice creating sentences. You want to be a better wizard? You learn the spells. You don't try to make a couple spells work in any situation. If you want to be a better writer, you do the same: you practice sentence structures so they're available when you need them.
Alongside these two beautiful spell books, I'd also have everyone read How to Write A Sentence And How To Read One by Stanley Fish. I've only just started the book, but I'm loving it. Fish provides the counterpoint to go along with the examples. He devotes an entire chapter to, "Why you won't find the answer in Strunk and White." Is it odd then, for me to recommend both? Not at all. Discussion and controversy inform us: our voice is what we make it. In balance, we find our own path; we take a stand with the structures we choose.
Fish and I are in agreement with much. He writes, "With each part of speech comes a list of errors you can, and probably will, make while trying to deploy it; obscurity of reference (what are these things, anyway?) is joined by fear, and the goal of being comfortable with the task of writing recedes into the distance." What a delightful show-off he is, creating that beautifully silly, long sentence in order to communicate a simple truth: if you make people feel stupid, you're not helping them learn.
I do believe that sentence structure is important---vitally important---to every writer. I do not believe in listing errors. Rather than spending all your time trying to fix already-constructed sentences, devote some weeks to getting directions, learning new spells, and playing. If you develop a flexibility in your means of expression, you will choose structures that best express your vulnerable, beautiful, rich understanding of life. You will not be changing your experience to fit the one or two sentences you know how to write. You see? Structure does not limit your voice; it allows you to express yourself more clearly, so your true voice---your true self---appears on the page.