Starvation, poverty, and homelessness are excellent motivators. Seriously. Even with a job, growth is everything. Your long term survival depends on staying vital, in touch with the tech and trends, and remaining interesting to your audience who may or may not be a future employer or client.
Slack and you stagnate. Stagnate and somebody else more interesting gets the job or assignment. Like I said, that empty fridge or stack of bills can motivate like nobody’s business. Studios generally don’t stick around forever and assignments eventually end. You, as a working artist, have to effectively be like a shark. Keep (it) moving or you die.
Honestly? There’s no surefire way to do that. However, keeping a journal of ideas or a sort of bucket list of dream projects helps. Getting through them? Incentivize, reward, and punish. In school there are consequences for failing to turn in assignments, rewards for success, and a reason (ie. diploma) to keep on going. Beyond that aforementioned general fear as motivator, establish a similar reward/failure system. It might be artificial, but it has been known to work in other endeavors where motivation and discipline necessary. Weight lifters and those trying to lose weight do that sort of thing. Couldn’t hurt.