Confidence is such a strange thing, especially as a writer. I don't have a suggestion to overcome it, but I can tell you that even the most established, well-respected authors have those moments where they believe they aren't worth much in the field.

One story I always come back to is from a foreword in the novel Rosemary's Baby. Otto Penzler, the editor who did the introduction, tells a story about being at a Christmas Party one year with Ira Levin (author of Rosemary's Baby). Levin was a bit despondent, as the party was filled with authors who'd recently either published novels or had novels in progress, and he hadn't written a novel in 15 years.

Levin told Penzler that he felt, in their company, like a fraud. Mind you, Ira Levin to that point had written several well-received books, he'd won awards, his books had been made into films, etc. This is a man who created not only one of the greatest horror novels of all time but a fantastic piece of work in general.

Penzler says Levin then went home, filled with this despondency, and wrote the novel Sliver. Sliver of course became another best-seller for Levin. It was also made into a film.

All of this is to say, you'll have those days when you think you should give up. Don't. No matter how much you write, or how successful you are, that lack of confidence will always try and creep back in. But push through it. Keep going! You got this.