I don't know that I have an answer for you, but I have been there. Several times.

My stories have a tendency to "shake down"- where I get a new, better idea that radically transforms the original, sometimes (usually) to good effect, but sometimes to irreconcilable difference.
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a friend of mine thinks you should always finish everything you start just so you have experience at finishing things as well as starting.
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This is a good point. To date the only story I've finished (beginning-to-end) is one where I forced myself not to retrofit new information into what I'd already written.