You need genuine passion to sustain hard work over a long period of time. A deep interest in the problem or the project.
It's why side businesses fail a lot of the time. Few people care enough about building businesses for their own sake to build one they don’t care about.
So the question is: what interests you enough? What are you passionate about? What is your aim?
If we knew the answer, we’d never struggle to choose a major or a first job. We’d never bother thinking about the question more than once.
Finding your aim is an iterative process. You start with your best guess, and course-correct from there. Over and over.
A useful technique is to shift your frame of reference. Instead of asking, “What is my purpose?”, ask “At the end of my life, would I be happy with what I’m doing right now?”
A similar technique is the Regret Minimization framework popularized by Jeff Bezos. Ask, “When I'm old, will I regret not doing this? Will I regret doing this?”
You won’t have a perfect answer. And the answer may change over time.
The important part is asking the question.
What is my aim?