1.
Stephen King wrote in his book “On Writing” about an Ideal Reader. For him it was his wife, whom he would bounce ideas off with, and be the first to read the manuscript. She was the litmus test for his writing. I think having a specific audience in mind is helpful—I find that I actually do my best writing in my letters (yes, I am someone who still persists in snail mail), when I have a very clear, specific feeling that I wish to articulate to some dear to me.
So first and foremost, I would think of writing as a type of communication, a need to express something specific. Form/Style comes from a desire to be as precise as possible with that need. Sentences aren’t long or short throughout the novel because you are trying to manipulate the reader, they are long or short because the feeling, or situation, or whatever you are trying to convey (excitement, suspense, loneliness, humour, etc) demands that rhythm.
2.
As for developing your own writing style. You basically need to read more, and write more. Read more of your favourite authors. Figure out some way to process the way they write (e.g., translate them, try to write a chapter in pastiche of Maugham, etc) In doing so you have an intimate sense of why they made the choices they did. If you like authors of very different writing styles, even better. After swimming in their language, gradually, you will figure out what works for you, and shake off what is unnatural (for you).
But above all, write more, write more of everything, and pay attention to what you write and how you write. When you write your question on fluther, is it in the clearest and most attractive form to this particular audience? Write your novel. Write short stories.
good luck-