I've allways had a problem with maths, especially mental maths under pressure. I still have this problem but I know it and therefor I can plan ahead for it. In the long run, I can make shure that I know the methods that are the easiset and that I know which type of calculations that will be made.
In the short term, it means that I make shure that I have all calculations done way before I need them. Descent calculations for example I try to complete just after leveling off for cruize and all other calculations to be done before the descend.
As for groundschool, if you can handle addition, subtraction, multiplication, division, basic algebra and handling of formulae, basic trigonomitry (SIN, COS and TAN), you should be well off. Navigation and aerodynamics is mostly mathematics and physics and it really helps if you can handle your numbers here.
Another thing that has helped me is to learn to see what is "reasonable" or not. This is not allways something to take for granted, but it will if you know how, help you in avoiding major mistakes by letting you see the bigger picture and get all the nubers into a context (such as choosing which direction to go to get to a destination, north or south...).
Hope this helps.
/LnS