Ask HN: Why is nearing completion so demotivating?
This has been mentioned in various permutations, and I'll add another one. The happiness in figuring a puzzle out is not when action is taken, it's when you figure it out in your mind. What about when you >think< you have figured it all out? Could you trick yourself into considering that your proposed solution might not actually be correct and you have to build it in order to check? Perhaps that doubt could slow someone down as well, enough to not want to continue either. 
What helps motivate me to finish is guilt, or more to the point the guilt of someone expecting the outcome and not receiving it. Not the best thing to chase/avoid, but everyone has their carrot/stick. Also consider that the work and upgrades are never over so the current release is just the latest iteration of your efforts.