I realized over the summer that I still didn't have a clear picture of who I wanted to be. Here's a procedure I used to help uncover the answer.
Make a list of a hundred or so people that seem interesting. Lean towards over-including. My list has parents, presidents, teachers, anime characters, founders, book characters, professors, writers, mentors, YouTubers, people I've only heard stories about, etc. Anyone who you know well enough to imagine a bit of their life is fair game.
Now, try each person on for size and decide if you want to be them. How would I feel about being MLK? My 18.06 professor? Mayuri from Stein's Gate? etc. Don't worry about justification; just try to feel out whether the sum of their choices, life, and lifestyle is innately appealing to you. Pay special attention to your gut reaction. If you put them in the "yes" category, you should be excited to live their exact life, flaws and all.
This process feels like trying on clothes, but instead you slip someone else's life over your shoulders and stand in front of the mirror of time.
Once you've divided the yes-list from the no-list, try to understand what separates the two. For example, I don't want to be Warren Buffet or several Jane Streeters I know, so I must not care about being rich. I don't want to be Linus Torvalds or Steve Jobs, so I must care about being nice to other people as a terminal value. I don't want to be Doublelift or Levi Ackerman, so I must care about working with other people more than extraordinary individual talent. I'd be happy to be MLK or Gandhi, so I must not care about technology per se. After some wrangling, you should end up with a set of things you care about.
The real value of this process is that it's hard to deceive yourself. For a long time I wasn't sure whether I cared about money. But after checking in with my feelings about living the life of someone spending their 20s working at Jane Street and driving a Lambo, I'm confident in saying that money isn't that important to me. It's much easier to decide confidently whether you want to be a concrete person with some values than to weigh half a dozen values against each other in the abstract.
Having confidence in your values gives you a lot of power. This October, I caught myself applying to trading internships. But this exercise makes it clear that I don't want my life to move towards trading. So I emailed my recruiters and cancelled my applications. It's one of my most self-actualizing moments in recent memory.