For me, song writting begins with being inspired, excited, amazed, or some similar emotion. At that point a lyric appropriate to the theme of whatever it was that excited me comes to mind. The first verse is always so easy. Then, I have to do a another verse, a bridge, a lick or two of fillers, and it becomes work. So I work at it til Im not excited any more, and like others have suggested, let it rest a bit. But sooner or later, you have to be true to yourself and finish the job. I love it when I can play a new thing for my wife, and then I hear her hummining it a few days later. She is my best critic.