I'm in a creative writing MFA program right now, currently applying to creative PhD programs, and I'm experiencing the exact same thing. Convinced that everything I've written, including my writing sample, is awful.
I'll admit it, I'm scared, but I just keep telling myself that even if I get rejected from all my schools, it's not as if I'll stop writing. The page will always be there. Rejection always sucks, but it's never the end.
So take a deep breath, don't be too tough on yourself, and know that there's another scared writer over here trying to do the same.
