I feel your pain.
I do German A-Level and it is without a doubt the toughest subject I do. I however am the opposite of you- I always forget the grammar rules but have a stunning vocabulary.
My advice? stick it out. You have a few months left, and you might surprise yourself. I thought I was going to fail out completely, drop the subject and forget about the dative, subjunctive and pluperfect, but I got a C. Considering my universities are asking for B's, I looked at it, dropped a subject where I hated the teacher, and am now on target for a B.
My advice with the vocabulary would have to be to use it. I used to struggle with vocab lists until my teacher started using a method by which she'd take an article, find key words within said article, put them into a vocab list [by theme, with the most recent being homelessness, for example] and then have us find the words in the text then work them out through the context. I find this helps, so maybe try the same in some way?
Learning words in list form never really works, in my experience so try it.
Viel glück =]