I have been writing professionally for more than two decades and teaching writing nearly that long. Every conference, every writing staff, and every class has always included some writer who asks: "How Can I Become A Better Writer?"
They don't usually like my answer. "Practice writing every day and read to study the writing of others every day."
They don't like the answer for one of two reasons usually. Some people are looking for some quick fix, magic formula that will make them a good writer in three easy steps. Although my suggestion involves only two steps it is pretty obvious that it is a long-term project. The other group of people will sneer at the first because they consider themselves "real writers" but they don't like the answer any more than the others. They believe they have a gift that simply needs to be unlocked by the magic key that published, successful writers possess.
The simple truth is that there is no way to improve your writing other than to continually practice your craft. Write every day. Experiment, plan, revise, and revisit. Make challenges, deadlines, and competitions. Push yourself and your writing will reward you. I promise. Write something inspired by a writer you admire and then write something completely your own.
However it is not enough to simply write in a vacuum -- or an ivory tower. You must also read the writing of others. Read far and wide. Read fiction, nonfiction, poetry and song lyrics. Read argument and persuasion, read informative and biographical, read science and fantasy. Read talented and skilled professionals and read those who are still finding their writing feet.
You are reading to gain inspiration and confidence. You are reading to build your vocabulary and your stockpile of writing tricks. You are reading to learn more about the rhythms and patterns of language. You are reading so that as you write you will be able to develop your own unique voice.
Learning to be a better writer is not the work of a weekend or even a semester. Learning to be a better writer is a life's work. If you really are a writer then you will never consider your work done. I don't know a professional writer who sits back and says "I'm done learning now, I'm as good as I'm going to get". Certainly it need not take a lifetime to reach professional status but you shouldn't make that your goal. Thinking in those terms can hold you back from becoming the best writer you can be.
For example, perhaps the reason your particular project was rejected had nothing at all to do with your writing but was in fact due to your topic, the particular needs of that publisher, or even the mood of the editor when your submission came across their desk. The truth is that you cannot control when you become a published, professional writer, but you can control your progress toward improving your writing. Believe me, the stronger your writing becomes then the easier it will be to achieve that other goal. When you reach the point that you regularly deliver quality writing then you will find a market. If you write it, the rest will come.