Well... the reason things go unfinished is not because you lack inspiration; it's because you don't finish them. If there's an unfinished piece of art on your desk, sit down, grab your pencils, and finish it. Simple as that.
As for motivation, you have to kind of train your mind. I've noticed that, at least for me, I'll feel really unmotivated to do like... anything; paint a picture, go to class, practice piano, anything. But, I've discovered that, even when I don't feel like doing something, after I actually start doing it it becomes a lot of fun.
So, in short: Just do it.
Yeah i know that feeling and that way too.
The problem may have its roots on the fact that i feel uncertain if i REALLY like do draw that picture. I begin to dout my interest for the sake of douting. My short: I dout if i like something which ive already started drawing.
PS. You could just ask me to use my feelings; do i feel good when drawing it or not? The Answer: I am uncertain if me feeling good is just illusion.
This "do i or dont i want to draw?" speculating is getting slowly on my nerves...