You determine what makes you you. What you're all about. How you live your life and what is most important to you. It is your identity.
Find out what your identity is. How? Look at your actions.
What do you spend your time doing? The collection of what you do makes up who you are.
Want a different identity? Change what you do. Create habits that create the identity you want.
It boils down to consistency.
Running one marathon doesn't necessarily make you a marathoner. If marathon running is so important, you'll find yourself doing it again and again and again. That makes you a marathoner.
What are you?
What identity do you want?
What are you going to do to get there?