It is not that we are what we think we are but if we only do what we think we can do we will never learn what we are truly capable of.

True.

But learning what you're capable of is also learning what you're not capable to do. You find out what your limits are and saying I can't or I am not is just stating the facts.

Take me and the (graphics) design as an example.

I've tried to learn how to make things look nice but I failed to do it. I've taken courses, followed tutorials,... no luck!

I'm sure that I could improve by investing a lot of effort, but it's just not worth it. IMHO, it's much better to play to your strengths than to spend a ton of time and effort in becoming a mediocre X.