It’s hard to feel fulfilled. As Josh Brewer and (through him) Frank Chimero put it, contentedness with work is like an appetite. It’s a constant battle between hunger and satisfaction. You don’t wake up one day, pumping a fist to proclaim, “I’ve done it!”
While this can be frustrating, it doesn't need to be. It just means being content is within our grasp on a daily basis. Simply enjoy the process. If you learn a new framework, celebrate it. When you finish a design idea, go for a long walk.
Contentedness is a mentality you choose for yourself. Looking outward to define your successes just doesn't make sense. It's all about your state of mind and acknowledging the wins you have, no matter how small.