How can I get a good grade if I just don't understand German grammar?

Check the mark scheme - when I did my A level, most of the marks were for content rather than accuracy. If the grammar rules are just not making any sense, focus on the actual points that you want to make, and learn by heart some key phrases that have impressive vocabulary and grammatical structure. By including these in written or spoken assessments, you will improve your content mark and give yourself a wider margin for error in the grammar sections.