I am capable of writing a novel. I have written novels in the past. I do have good ideas that make good stories. The first draft does not have to be perfect. I can do this. These are the things I should be telling myself, what I will start telling myself. And hopefully, the mean little voice will go away.
My advice to myself is this: Writing is what you love, so let the characters take you on this journey and enjoy the ride. Stop worrying about doing it wrong. Remember that art is subjective.