When deciding how to spend your time, I recommend you take into account two criteria:
Whether what you're doing can change the world.
How much you'll learn.
Even today, this is how I decide how to spend my time.
Our society today is incredibly good at giving individuals the opportunities to change the world. With digital technology and modern communications, ideas and products can spread faster than ever before. With the right ideas and strong execution, any person can quickly help a lot of others on our planet.
So, ask yourself: If what you're working on succeeds beyond your wildest dreams, would you have significantly helped other people? If not, then keep searching for something else to work on. Otherwise you're not living up to your full potential.
Second, especially when you're young, don't estimate the value of investing in your own future education.
My definition of "young" is anyone less than 100 years old.
Anything you learn will pay off for decades to come. But it won't be easy. Once you're out of school, investing time in learning has relatively few short-term rewards. There's no teacher standing over your shoulder to give you a grade or motivate you to keep studying. But if you can inspire yourself or make it fun to keep reading, keep playing with ideas, keep talking to people that you can learn from, then over a span of years you can become incredibly talented in your areas of study.
For myself, I love reading. I have >1000 books on my kindle, and spend a lot of time in the evenings and weekends reading. My reading diet includes academic research papers, books on business strategy, the innovation process, products, biographies of people I admire, and more. I sometimes take MOOCs. I also love talking to people who can teach me new things, whether an old friend or a new acquaintance.
The process of learning will also help you decide what to work on. When you've seen enough examples of what others are doing to change the world, you'll also get more and more ideas for how you can change the world yourself.
To summarize: Keep investing in your own learning, even when it's hard. And keep searching for a way to contribute to something that helps humanity!