I talk to a lot of people who are very, very sure of what they can't do in their careers.
They can't be writers. They can't be artists. They can't be travel agents. They have a giant list of can'ts. It's often larger than their cans.
When I talk to such people, I also learned very quickly that the majority of them really have little or no awareness of the career they've given up on. They hear something, read something, or make an assumption, and then kill off their career plans right there.
The only way for you to decide you can't do something is to know how it's actually done.