The most basic question every
Ph.D. student must know the answer to is: "Why the hell am I
If you do not have an acceptable answer to this question, then don't get a Ph.D. I repeat: if you do not have a rock-solid reason for getting the Ph.D., then it is better that you leave with a Master's.
Scientific writing is an art. It is the art to make clear to the largest audience the most complex ideas. It is hard, it is counter intuitive, it must be learned. You can have a look at my course material on The Art and Science of Writing. How to write easy-to-read text? The two following references are also very good.