If this art business were a train, discipline would be the wheels. But to get that train moving, you need an engine - motivation. If demand for your work is low, it might be hard to stay motivated. Maybe you're just starting out and haven't built up a client base yet. Or maybe you've been doing art for a long time and your clients don't have any wall space left. Or maybe your clients are wrestling with an uncertain economy and just aren't buying. You might feel like giving it all up and going back to your day job.
Words from experience speak the loudest.