Think of what you write as a draft. One day, you'll write that final draft--maybe about the time that you're accepting the proofs of a work set for publication--but for the most part, what you're doing is writing drafts.
Each time you set the pen to the page, what you write is a test run until the next time that you attempt to face the same question. You're writing it to see how it works out. You have an idea that you want to express, and you also have some idea of a means of presentation, and you try to combine those two into a working piece of writing. And sometimes it works.
When it doesn't work, though, what does that mean? Well, it might mean that you got negative feedback from someone who panned our work. Or it might mean that you yourself don't like it. But what do those mean for the future? Do they mean that you should give up, and that's the end of your writing career? Of course not. All it means is that you go back and write another draft.
Maybe the feedback you got was a rejection without any discussion of reasons for rejection. That's probably the toughest. Then you have to decide whether you believe in the project and whether you think that what you have done is the best you can do.
Alternatively, you can look for ways to do it better. If the draft doesn't accomplish what you hoped, you can go back knowing, at least, that the old draft itself was not enough.
Ideally, of course, you work is not simply rejected out of hand, but rather any rejection is accompanied by cogent feedback on what needs to be changed. Dissertation writers reasonably can expect to get some cogent feedback from their readers.
Feedback is invaluable. When you have written a draft, feedback can guide you to the next draft.
Don't be afraid to write, or to rewrite. If you write a new draft, don't hold too hard to the pieces of the old draft: they carry with them the traces of old ways of thinking about the project, and can distract from creating a coherent vision that matches your most recent experiences and studies.
You always want your drafts to be complete; you want them to be careful and as coherent as possible. You want your drafts to express the whole scope of your ideas. You do not want your draft to be perfect, free from all errors or spotless; that should be saved for the version that is already been accepted. The effort that goes into making a draft completely free from punctuation and grammatical errors is too great to be expended on a draft that may be significantly rewritten. Though obviously any draft should be clean enough to be readable.
You may be in a situation where your drafts are required to be grammar-perfect; that's unfortunate, thought there's not much that can be done about that, except put in the resources to see that it is grammar-perfect. It's unfortunate from the perspective of thinking about your draft as a draft. The more effort you put into any single draft, the more committed you are to it, and that is counter to the principle I'm espousing here: don't be too attached to a draft! Every draft is an attempt; it's a venture into the unknown waters of feedback; it's an exploration, but the most likely dangers are emotional. For dissertation writers, if you make a complete draft that covers the necessary issues and has addressed previous feedback, then what real danger do you face beyond needing to keep working on improving the draft? Are you really in danger of being kicked out of your program? (Yes, I do know that some people do get kicked out of their programs for not completing enough work or not doing good enough work, but programs want to graduate people; they don't want to flunk them out.)
The point of calling them drafts, perhaps, is to find a balance--a place where you both strive to make the draft good and where you're remain indifferent to whether it is seen as good when it is done. The process of completing the project is what is important, and as long as the draft helps you move towards that completion it has accomplished its goal.