Remember how outraged Hermione was when Professor Umbridge revealed that she would only allow her Defense Against the Dark Arts students to read defensive magic theory rather than actually practice it? That’s the same way you should feel about creative writing theory – if you aren’t putting it into practice!
You can read the sage advice “make your characters vivid” over and over again until you are repeating it in your sleep, but unless you study concrete methods to make characters vivid and practice these methods, both in isolated exercises and in your actual writing, your characters will never, ever be vivid, except perhaps by pure chance. That’s because we don’t learn by having information flashed in front of our eyes for a few seconds. We learn by practice.
You also need to study an instructional book that adopts an actual philosophy of writing and craft. I highly recommend any book by Jack Bickham if you are looking to write fiction. Books such as Zen in the Art of Writing by Ray Bradbury are invaluable, but they do not teach the strict matter of craft.
And, no, sticking to writing principles doesn’t imprison you. On the contrary, it frees you to unleash the magic of Story. Just like there are principles to building a car, so there are principles to constructing a good tale.
My favorite book by Jack Bickham is Writing Novels That Sell. This specific line from the book’s introduction kicked my butt into high-gear: “Here and there you’ll find suggested assignments and exercises. You can, of course, speed-read past these. You’re free to waste your money any way you wish.”
Jack Bickham taught my own writing teacher, and his book is revolutionary in its brilliance. I also recommend Deborah Chester’s The Fantasy Fiction Formula (Great quote: “I firmly believe that there should not be a veil of mystery draped over the creation of viable stories”). Even if you write fiction outside of the fantasy genre, this book will be immensely helpful. Deborah Chester was my writing teacher. She taught Jim Butcher, also. Anything by Dwight V. Swain is also excellent (he was Jack Bickham’s writing teacher).
One reason I found Professor Chester to be so helpful and engaging in college (and the same reason you will find her writing instructions, and those of her teacher and his teacher, to be useful) was that she based her feedback on the writing craft she had already taught me. I can rattle off much of that here: scene structure, characterization, stimulus and response, climax, and motivation (there is much more).
Never let someone who is not proficient in the skill of writing craft and story principle tell you that you are being rigid or uncreative by sticking to effective principles of writing that have been used since the days of Homer.