Firstly; breathe - I went from a D to an A* in the space of a month (roughly)
I will say the best method is using past papers. This means simply, completing a paper and using the solutions to understand where and why you went wrong on a particular question. If you understand it, you will be fine.
If you're performing at a D grade and want a C grade it stands to reason that you do the foundation paper, which will be much easier. A D-grade on a higher paper is almost certainly a C grade on a lower paper.
Good luck - any questions feel free