Maybe you’re not born brave.
When you see an unjust fight, you don’t instinctively get involved.
When you see a girl (or a guy) you like, you don’t walk up and introduce yourself.
You’re not a coward, but you can’t rely on that factory-default courage that some people seem to have.
So you need to find other ways to be brave.
The easiest way to find bravery is to burn your boats. Block all exit routes.
Make sure you have no way out. No escape.
No matter how timid you are, not having any options will give you the courage to do what has to be done.