Your supervisor will be your principal point of call but you should also try and cultivate other sources of feedback, as much to develop your confidence as to garner information. Attend (or set up) postgraduate progress seminars where you can read each other’s work.
Acknowledging and accepting that postgraduate research in all its constituent parts - developing a research topic, building an argument and learning to write - are all extremely challenging tasks and will take a while to get to a high standard may help you to detach from your work. Many people identify with their work and take criticism very personally. You don’t have to: you are not your work (although it may be changing you). Take criticism on the chin and see whether it can be helpful. Ask yourself the following questions: could the criticism possibly be justified? If so, how? What is the underlying problem? What can be done, how and when? Once you’ve responded to all these points, you’ll have a fully-fledged action plan, and not feel so bad. So good luck! Get writing! Get re-writing! We look forward to reading it.