Finishing is a skill, and most people only ever practice starting.

Starting is pleasant and finishing is not, which is why most people have far more experience of one than the other. The last ten per cent of anything is where the boring problems live: the fixes, the decisions you deferred, the admission that it will not be what you imagined. That stretch has its own techniques, and you only learn them by going through it repeatedly.

Finish something small this week that you would normally leave at eighty per cent. Accept a version worse than the one in your head and call it done. The discomfort of that is the practice, and it gets easier by the fourth time.