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If your hero can't possibly win, then don't have him win. Have him survive, but just barely! Then apply all of that effort which you were willing to invest in a total rewrite, into writing a sequ...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/27285 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
If your hero can't possibly win, then don't have him win. Have him survive, but just barely! Then apply all of that effort which you were willing to invest in a total rewrite, into writing a sequel. In this second volume, you can strengthen the hero and weaken the villain or even change the nature of their conflict into something else (a.k.a. introducing a bigger villain to unit the former enemies). Don't force your book into a fairy tale ending, just because it has gotten to the length where most stories end. Instead, take your characters through some unexpected plot twists; find an alternative victory for your hero and leave your villain's final fall for some later tale. You've spent your entire novel creating and fleshing out two wonderful characters. Seems a shame to not keep them both around for your next book.