A lot of times, we are faced with patients and clients (and our kids) who will say "I can't do it." They do not have confidence in their ability to do something, and that makes it harder to try. This may be more often encountered in kids with ADHD, where due to distractions tasks take longer, are harder to complete, and the kids are more likely to make mistakes doing the tasks. Doing math is a place where this happens a lot. One careless error can upset a whole math problem and make kids feel like they are not good at math--and then they say they can't do math--and then they don't.
When your child says "I can't do___" ask them to end the sentence with "YET." In the book, we talk about how YET stands for Yes, Energy and Time. Because if you can get your kid to say Yes, with Energy and Time they will be able to do the task. "I can't do ___ YET" acknowledges their current difficulty, but implies that they will find a way to do it. With belief that they will do the task, they are more likely to do the task. Y.E.T. takes a negative and turns it into a positive. Not bad for three little letters!