Hi Pat,
Firstly i would say scrap the instructor, and find a new one with the patience and decency to help you along the course at your pace! Secondly dont bring yourself down worrying about your maths skills or anything like that, I personally have just short of 14 hours in my logbook and i cant think of any maths i have been asked to do. Additionally dont take critisism from him on your maths. Everybody has different abillities when it comes to mental sums and what gives him the right to comment on your abillities in such a way.
If you are at the point where you are not enjoying going flying something needs to change. Its not going to be you, over time you will develop your maths, your speed with checklists etc, but for now your instructor needs to change. Because by the sound of it he is there for the £££ more than wanting to help a determind student pilot grasp the world of aviation!
Ditch the instructor and enjoy flying once again; at YOUR OWN PACE, not the instructors.