If it helps, I used to be motivated out of sheer wanting to get that thing that was in my head out, and see it working. That might be enough for a youtube video or a product demo with some executives at the company. Heck you can turn that 80% into a real career booster (I've done this), but it's not a production ready product. The last 20% of the work will take 80% of the time. It's really hard to be motivated for that last part. It's all the hard stuff that seems insignificant... and you've already got the cheese.

If you can accomplish it, the trick to staying motivating is to figure out the mental gymnatics to move where you put the cheese in the trap. If you can do that... you have a real chance at being motivated past the proof of concept.