I don't think I ever finished a single map ever. (well, i finished a few small some that kina sucked.) I also suffer from "heck. don't give a damn anymore" quite often.
Actually, I don't even remember all project I have "hanging around" ATM. time to check.
*checking*
OOO totally forgot about this one...cute...but too big for me...
So here is my advice (its the only way I get things even mostly done): split your project into multiple stand-alone projects, each one does not NEED the others to be perfectly playable.
Once all "parts" are complete you jest need to combine all into a single map, and then add anything new that might be needed from the combination (for example, if you spilt your risk map to a few territorial-based sub-project you might need to make transportation of correction to the terrain to make everything fit in. but in general they are all done.)
This also allows for the "well. that's a mistake, lets take that part out" to be done with ease.