There is no easy fix for this. You need to be disciplined enough to keep working when you've done all the fun parts.

The good news is that if you can get better at being disciplined just by practising it, and the rewards are very high, because you become a finisher, and people like and value finishers.

Also, the act of finishing is immensely enjoyable, and is therefore its own reward. With practice your mind will begin to be motivated by that reward, once the little rewards of the early parts of projects have gone.