A PhD is not, primarily, about writing; it is about research. The writing serves merely as a means of presentation.
That isn't to say that writing is unimportant, but a lot of people assume that because the final submitted product is a piece of writing, that writing is the primary aim or that doing a PhD and writing a thesis are the same thing.
So I meet students only a few months away from the deadline who have spent a huge amount of time writing, but have never even looked at their own data and have no experience or skill at all in analysing, presenting and discussing research results.
They may have written hundreds of pages, and those pages may superficially look like thesis chapters, but they have verylittle in terms of research content.
You might be a fantastic writer, but if your research sucks then you cannot write a good thesis. But if your research is good then the writing doesn't have to be fantastic; just good enough to explain what you did and what the results were.
Do the research. Do the analysis. Discuss your results with others and get feedback. Repeat. Then write!