Self-confidence is a great tool. The last few days I have felt that I was writing a masterpiece of criticism, a great trilogy of Lorca criticism, that I could do what I was doing well and that nothing would stop me (despite some recent turmoil in my personal life).
Curiously, the renewed self-confidence resulted simply from returning to work, after a few months in which I wasn't producing a lot or even trying that hard. My self-confidence seemed to slip during those months. No mystery there. Cut off from the source of my strengths, I began to doubt myself. Sure, I had been able to produce before, but would I ever do so again? Even though the break was a deliberate one, in some sense, and I knew intellectually that I would be able to resume when I wanted, the effect was predictable.
Of course, if you're anything like me, you will be doubting yourself with some frequency. Isn't the book I'm writing just a repetition of what I've done before? Aren't people going to reject this, or be uninterested in it, because most people want to follow the dominant paradigm of Jo Labanyi Spanish cultural studies in which poetry itself is insignificant? Writing another book or two won't get me a better job / salary / a distinguished professorship here at KU, etc... Maybe I don't write well enough in Spanish to write my book about Lorca in that language?
Doubt is useful, I guess, but I would advise that you address doubts as pragmatic problems to be resolved rather than as existential ones. In other words, if you have a nagging doubt, bring it out into the open and see what's going on. A generalized doubt like "I'm not good enough" is largely useless, because it has no practical solution.
Self-confidence is not arrogance, but the necessary fiction needed to get the work done at all.