overwhelm during a challenging task

– C studying a math book to absorbe the concepts
– T this is too hard to understand
– F overwhelm
– A grit teeth, salivate
– R additional suffering on top, make the learning experience harder

I am trying to come back to the present moment and to focus on the next thing to do, and telling myself that I can understand this. However my brain is going back continuously to “this is too hard to understand”, and apparently it is believing it. Any suggestion? Thanks!