Never say you can't because everything you say happens, even sarcastically, it can happen, for example if I say "I'm stupid" you are telling your brain that you are stupid, as your brain is not capable of seeing what is happening it believes it so you're going to be stupid, so if I say, "I'm capable of doing a lot of things and I'm very smart" your brain believes it and you're going to be smart, it's all a matter of practice.