Motivation takes many shapes.
When it comes from within (also called intrinsic), you will want to do something simply for the sake of doing it (e.g. having fun, having a purpose). To build on Lucian's answer of offering your work to real clients, maybe some non-profit cause strikes a chord in you.
When it comes from outside (also called extrinsic), it is more closely related to exterior factors (e.g. because it will make your life easier in some way, getting paid, getting recognition from peers, getting reputation points on a Q&A site...)¸
It is usually considered that intrinsic motivation is more powerful because it doesn't rely on anything else than your own self-drive. However, it's possible to build intrinsic motivation in time, by starting with extrinsic rewards.
Considering this, I think you need to do some introspection on what would motivate you most and build from that.
A few leads:
Intrinsic: What type of exercise do you find most fun? What kind of topics? What kind of medium?
I know that personally, I have a lot of fun learning new things and trying stuff I've never done before. I'll go from web design to silkscreening to photography and be happy. Part of these newly acquired skills transfer to my work.
Extrinsic: What outside factors do you think would motivate you most?
Do you think you'd be more motivated if you got paid for your work (granted having a real client also comes with other aspects that are sometimes a lot less motivating than just the money)?
Do you crave recognition or at least some kind of interaction with peers about your work? Then maybe working on shots on Dribble or setting a goal to reach on Behance could benefit.