I constantly doubted myself in grad school. The best cure is to get out into the real world. For now, just know you're perspective will change later. The confidence will come. You're already much better than you know.
Graduate students are well trained to criticize existing research.
They are skillful at taking apart even the classic work in their field. But they are relatively untrained in how to free the creative spirit that is essential in the early stages of writing. It's so easy for your Critic to tell you that your raw draft is flawed and worthless. It's so easy to imagine your committee tearing you apart as you've done a thousand times to research that was good enough to be published in refereed journals. It's so easy to freeze.