I have that same issue. The best thing you can do is save it so maybe later you'll be inspired to finish it or practice speed drawing. Speed art is the best way for unmotivated artists to produce semi finished work.
Or not sectioning a drawing into different days. IE Finishing it all in one sitting. Which takes a huge toll on the mind but can be so worth it in the end.
[spoiler]http://s1053.photobucket.com/user/palio7/slideshow/Art%20Portfolio?sort=3[/spoiler]
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Oh trust me I know, I've got a leather binder dedicated to stuff I whip out on impulse... the real problem is that it is difficult for me to focus on one piece because I always want to put make art around how I feel at the time. [spoiler]Yet.. taking that trip through your portfolio just motivated me at least until the end of the day. You have a very interesting spectrum of taste, that I personally like the most aside from the actual talent.[/spoiler]