Math skills build upon previous math skills so when children haven't mastered a math concept or skill set, it impacts their confidence and understanding. I've met many students who believed they couldn't "do math" when they only needed some instruction in math skills they missed along the way.
When I'm teaching math, I constantly give students short assessments (two problems often do the trick) to see what they know. I don't want to spend time teaching them what they already know. What I want to do is find the gaps in their knowledge and help them bridge those gaps with understanding. I do this by using games and engaging activities, along with skill-based practice, to help them master math.
It doesn't take long before students start to believe, then know for certain, that they can "do math" quite well!