Even if the scope does not include all the possibilities you can always go back and complete a second smaller follow up project.
Completing something is better than not completing anything, especially for ongoing motivation.
Small tight packets of work are very productive due to the marriage of focus with short term achievement.
If your projects are stalling, or just seem to keep on going without ever coming to a conclusion, then review / revise the scope and 'put them to bed'.
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