If you sit around and wait for the mood to hit, you might never write again. 

Push aside that idealistic image of the perfect combination of circumstances you imagine yourself sitting down to write at. It may never come and that tale in your head may never see the light of day.

You have to make it happen.  J. K. Rowling used to write between classes and over lunch. She did what it took to write her stories.

You have to fight for time to write.  Are you up for it?