The first thing, when one is being worried as to whether one will have to have an operation or whether one is a literary failure, is to assume absolutely mercilessly that the worst is true, and to ask What Then? If it turns out in the end that the worst is not true, so much the better: but for the meantime the question must be resolutely put out of mind. Otherwise your thoughts merely go round and round a wearisome circle, now hopeful, now despondent, then hopeful again - that way madness lies. Having settled then that the worst is true, one can proceed to consider the situation.
From The Letters of C.S. Lewis to Arthur Greeves
but found, as usual, in: