I feel in the same situation as you at the moment, I need to really raise my level of maths in the subjects of mechanics and finite element methods. I must admit I am not a natural mathematicians. I am also having to go back to basics for some subjects to build up. I also strugle to remember the stuff I don't use often, but I think this is natural.
Actually this is one of the reasons I joined this forum too. I hope I can find a solution for both of us....
All the best