You don't need self-confidence.
You just have to do it.
A story in the Calgary Herald about author William Neil Scott proves this point. He was studying at the time--the article takes up the story in his words:
"That whole year in this manuscript course I was trying to finish the book and I was not enjoying what I was doing, so I would keep switching to different books until I had two weeks left. And I wrote Wonderfull in like a week's stretch. It was half the size that it is now. I finished it the morning of the class. I handed it in thinking I was going to fail the class. But that was the first real solid grade I got. I finished something."
The article says, "Not that this did the trick in boosting his confidence. It took persistent urging from [his teacher] van Herk for Scott to send the manuscript into NeWest Press, a small Edmonton publisher that immediately accepted it. Again, Scott seemed less than enthusiastic. "I remember talking to them on the phone and I said, 'Really? You want to buy this book?' " Scott says. "I said, 'I can do so much better. I will rewrite the whole thing.' And I did."
When the book was published, it got excellent reviews and was nominated for several prizes. But he said he still felt like an imposter. Now he's working on his next book and he dares to think that maybe it'll turn out OK.
I encounter a lot of writers or people who would like to write who say they lack confidence in their writing and that's stopping them. This story underlines the fact that you don't need to be confident. You just have to keep writing.
(for encouragement and useful guidance, see my book, "Your Writing Coach," available from Amazon and other book sellers.)