I came across this discussion of doubt in a book called Letters to a Young Poet by Rainer Maria Rilke:
Your doubt can become a deep attribute if you discipline it. It must become a knowing; it must become the critic. Ask it, as it always wishes to spoil something, why something is ugly. Demand proof of it, test it, and you will find it perhaps perplexed and confused, perhaps also in protest. But don’t give in; demand arguments. Act with alertness and responsibility, each and every time, and the day will come when doubt will change from a destroyer to become one of your best fellow-workers, perhaps the wisest of that have a part in building your life.