Being able to do something is often a matter of perspective.
There are many things that seem impossible, but some people can do it. Are they magician? No, they simply figured out how to do it.
When we say “I can’t do it” beforehand we’re expressing our little hope in the outcome, the fact that we feel like our skills don’t match the test. We feel lower than expected.
But if you see it this way, then you just decided what you can’t do. Not because you can’t, but because you don’t believe in it.